Hey all. A while back, I decided to try to wire the fog lights on my '81 300D (US model) to work more like a Euro model. Here's a link to that project: Fog light wiring modification (US wiring to Euro wiring)

I continued to mod my car, and decided that I'd might as well try to get the rear foglights working on it.

Anyway. I was quite successful in getting the rear fog light working on my '81 300D, right down to the indicator light in the headlight switch. Here's how I did it!

As usual: If you decide to take on this project on your own car, you are doing so at your own risk. Please look into your local laws to see if such a modification is legal in your area.

Tools/supplies needed:
-Electrical diagram
-Phillips screwdriver (magnetic tip is REALLY handy here)
-Flat head screwdriver
-7mm nut driver
-24mm socket w/ ratchet
-Wire crimpers
-Wire strippers (mechanized ones are a plus)
-M4 x 6mm screws. Qty: 2 (good idea to have more, in case you drop/lose some)
-M4 lock washers. Qty: 2 (good idea to have more, in case you drop/lose some)
-Good quality electrical wire (I used 14-gauge).
-Ring terminals. Qty: 3 Use correct size for M4 screws, and the wire you'll be using.
-Electrical tape.
-Soldering iron and solder.
-Male and Female "bullet" electrical connectors (I got these from a junkyard 300TD)
-Bulb socket for rear lighting bulb holder. Qty: 1 or 2, depending on how you want to do this (I got these from an abandoned 240D. You can also find these on ebay in a "brake light upgrade" package.)
-A Sylvania 2721, Osram 6.5V 1.5A (or equiv.) bulb. (This is the same bulb as used in our instrument cluster lighting.)
-Sylvania 7506, Osram 12v 21w, (or equiv.) bulb. Qty: 1 or 2, depending on how many rear fog lights you want to have.
-Some patience, and perhaps a good friend to lend a helping hand.


Step 1: Disconnect battery (no pic, sorry...)

Step 2: Remove driver's side knee bolster. It is held in by 3 screws near the top, and 3 plastic screws on the bottom corners (no pic, sorry...)

Step 3: Remove headlight switch. You'll need to remove the knob first - simply give it a solid tug, it should come off. Use a 24mm socket on the big nut that holds the switch in place.



Step 4: Remove driver's side floor mat/carpeting and foot rest.



Step 5: Locate wire harness for the rear lights. Remove wire harness connector from its bracket by pulling it towards the rear of the vehicle until it stops, then pull it towards the passenger side to release it from the bracket.



Step 6: Separate the two connectors. Use a flat head screwdriver to remove the caps from the connectors to expose the wires inside them. I will refer to the tail light side of the connector as "rear", and the headlight switch connector as "front". On the rear connector, we'll be removing the white/green wire from the male bullet connector (or pin) which it shares with a gray/purple wire in location #9. We'll solder the white/green wire to its own male bullet connector, and put it in location #11. Euro models have the wire for the rear foglights in location #11, so that's why I chose to relocate the white/green wire there.




I used the pliers only to hold the bullet connector while I was desoldering the wires from it.





Step 7: Solder a female bullet connector (pin) to a new wire. This wire will run from the "front" half of the wire connector to the NS terminal of the headlight switch. Cut it to length, and use a ring terminal on the opposite end. Place the bullet connector in location #11 of the front connector. The wire I used was red.



Step 8: Run the new wire (from the previous step) to the headlight switch, and screw the ring terminal into the NS terminal. You may need to disconnect a wire or two on the headlight switch to gain access.



Step 9: Run a ground wire to the headlight switch, terminal 31. This will be the ground source for the indicator light. I used a brown wire to keep it consistent w/ the other ground wires on the car. I grounded the wire using the hold down bolt (use 7mm nut driver) for the warning buzzer. Secure the wires, as needed, to keep them away from the pedals, steering column, etc.





Step 10: Snap the covers back on to the two halves of the wiring connectors. Connect the two halves together, and place it back on its bracket. (sorry, no pic...)

Step 11: Reinstall the headlight switch, knee bolster, and floor mats. Install the indicator bulb (Sylvania 2721 or Osram 6.5V 1.5A, or equivalent) in its socket on the headlight switch, then reinstall the headlight switch knob. You're done with the under dash wiring!





Step 12: Open trunk, and remove/unplug the left side tail light bulb holder. It is held in with two round plastic knobs. Carefully unsnap the metal bulb holder bracket from the plastic piece, to expose the bulb sockets and wiring.



Step 13: Snap in a bulb socket in the empty rear fog light hole. Desolder the green/white wire from the sidemarker bulb socket, then run this towards the new bulb socket for the rear fog light. You might need to extend this wire, as mine was too short to reach the rear fog light socket. Solder the wire to the new rear fog light socket.





Step 14: If you want the sidemaker bulb to continue working (for legality or safety reasons), you will need to solder a jumper wire from the sidemarker bulb socket to the running light socket. Omit this step if you have Euro tail light lenses. I used a black/red wire for this.



Step 15: Reassemble the two halves (metal/plastic) of the tail light bulb holder. Install a new bulb (Sylvania 7506, Osram 12v 21w, or equivalent) in the new rear fog light bulb socket. Reinstall on vehicle (sorry, no pic...)

Step 16: Repeat Steps 12-15 on the right side. If you choose not to have a rear fog light on the right side, you MUST desolder the green/white wire from the sidemarker and isolate it. The green/white wire no longer serves as the sidemarker wire, it is now the rear fog light bulb wire.

Step 17: Plug the battery back in, reset your radio/clock, and test the rear fog light for functionality! Note: I wanted to have two rear fog lights, but I accidentally broke the bulb holder for the right side. So for now, I'm just running the rear fog light on the left (driver's) side.








Another pic with the brake lights on (yes, I know one of my license plate lights is burned out, lol):



And the next day, I fixed the rear foglight on the other side:


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Darren Asuncion
'81 300D Silver Blue metallic
'04 MINI Cooper S, Indi Blue w/ White roof
'94 Jeep Cherokee Sport, Flame Red

Although I have decided to use my rear fogs as extra brake lights this is a concise and well documented write up. Neat, tidy work and looks good.

Thanks,

Jay

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1984 300D turbodiesel
1983 300TD

I'm curious - do you still have functioning "standing" lights?

On my W140, turning the headlamp switch nob the opposite direction from normal (don't remember which way that is) activates the front and rear parking lights first on one side, then at a second detent, the other side of the car.

I think they're referred to as standing lights - not much use here in the US, but apparently useful back "home" for these cars.

Regardless, great "reuse" of existing infrastructure to reactivate what I consider to be a very useful feature. I was unaware that there was a similar bulb on the passenger side - I think my W140 only has a socket on the driver's side.

Oddly, my W140 ('92 300SE) has the ability to run the fog lights with the parking lights (don't know about the hi-beams), and the rear fog lamp is functional as well. I was surprised more by the working rear fog lamp than the parking light-enable fog lamps - all my previous experience with MB cars had the rear lamp non-functional.

The only downside to this is that when you use the standing lamps it will run the battery down quicker because the side marker lamp is running as well. But IMHO it's worth that loss to not have to run a new wire all the way to the back of the car!

Instead of connecting the side marker light to the rear running light, why not connect it to the license plate light circuit? Then it would function in the OEM way.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Hey thanks!

Something I thought about doing - why not have the best of both worlds? It wouldn't be too hard to have both, once you've gotten the wiring done for the rear fogs. All you would need to run, in addition to the brake light jumper wire to the rear fog light, is a diode.

The idea: When you hit the brakes, both the brake lights and the rear fog lights will come on. In the event you ever 'need' or want to have your rear fog lights on, the diode will prevent current from the rear fog light circuit from turning on your brake lights too.

I actually bought some diodes, in case I decide to do this. It wouldn't take much time at all, since you'd only have to tear into the tail light bulb holders...

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Darren Asuncion
'81 300D Silver Blue metallic
'04 MINI Cooper S, Indi Blue w/ White roof
'94 Jeep Cherokee Sport, Flame Red
'95 Land Rover Discovery, Corallin Red

This is definitely something I've thought about as well. I rarely ever use the standing lights on my car, so I figured this wouldn't really be an issue. I'm actually looking at running Euro tail light lenses on my car (if I could ever get a set). When I do switch over to the Euro lenses, I will be removing the sidemarker bulb.

The reason why I didn't run it to the license plate light circuit - I was avoiding running any additional wires. My solution minimized the extra wires, and more closely mimics how the Euro cars were wired (according to the diagrams I looked at). Euro cars do not have the sidemarkers to my knowledge. The method I used to keep the sidemarker functional restricted the 'extra' wiring to the tail light bulb holder: quick, easy, and is nearly undetectable when the car is put back together.

If you are in an area where you'd regularly use standing lights, yes, I'd definitely consider wiring the sidemarkers to the license plate lights...

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Darren Asuncion
'81 300D Silver Blue metallic
'04 MINI Cooper S, Indi Blue w/ White roof
'94 Jeep Cherokee Sport, Flame Red
'95 Land Rover Discovery, Corallin Red

Yup, my w123 has functional standing lights - from the factory.

I've since converted my front lighting to Euro-spec, right down to how the corner lights operate. What can you say, I'm trying to Europeanize my US-market car. So now, instead of having the corner lights illuminate with the parking lights/standing lights - the "city light" inside the headlight housing illuminates.

I'm pretty sure that Euro market w123s only have the rear fog light on the driver's side of the vehicle. Mercedes included the openings on both bulb holder brackets, presumably so they could easily adapt for LHD/RHD vehicles w/o having to make market-specific bulb holders. Something I'd like to look into - I've always wondered if Euro spec cars were prewired to have the rear fogs on both sides. On some other Euro cars I've had in the past, bulb holders existed on both sides, but the bulb was only installed on the driver's side. I wonder if Mercedes did the same thing...

I *think* the reason why later-model Mercedes have functional rear fog lights might have something to do with certain states (like Alaska and Washington) allowing the use of the rear fogs in later years. The last I looked, there were more states that allowed the use than not.

In any case, I'm annoyed with how our w123s come "crippled" w/o rear fogs, and the ability to run the foglights with the parking lights or high beams. I do understand why some laws here dictate that fog lights are not to be used with high beams (not supposed to use high beams in fog, among other things), but I feel *I* should be able to choose as long as I don't harm or endanger others in the process. There ARE times here in Alaska where running fog lights + parking lights allows you to see MORE than fog lights + head lights...this must be safer, right?

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Darren Asuncion
'81 300D Silver Blue metallic
'04 MINI Cooper S, Indi Blue w/ White roof
'94 Jeep Cherokee Sport, Flame Red
'95 Land Rover Discovery, Corallin Red

Just don't use your fogs as regular night driving lights now. Those bright rear lights are a real nuisance when I'm in traffic and the bozo in front has his fogs on. It lights up the interior of my car even when I'm a full car length behind.

Plus without a central brake light, people may think your brake lights are stuck on.

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1984 300TD

Thats reason enough to not wire them as rear fogs. Even is areas with common fog, most people won't know what they are. They will just figure you're riding your brakes.

I never use the rear fogs on any of my vehicles unless conditions warrant it. It is annoying when people turn them on needlessly - they are bright!

If I'm in heavy traffic (in the fog), I don't normally turn them on. In heavy traffic, speeds are relatively slow, cars are really close together, so visibility isn't a problem.

I'm in a rural area, where fog can set in really thick, and unexpectedly. I like to use the rear fogs in these conditions, just so someone coming down the road behind me is more likely to see my car. I'll shut off the rear fog lights as soon as someone else is behind me.

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Darren Asuncion
'81 300D Silver Blue metallic
'04 MINI Cooper S, Indi Blue w/ White roof
'94 Jeep Cherokee Sport, Flame Red
'95 Land Rover Discovery, Corallin Red

That's why you turn them off as soon as someone gets behind you and can see your car. It's pretty rude to just keep them on, and people are more likely to become immune to seeing your other lights if you keep them on.

Rear fogs are most useful if you're one of few vehicles on the road on a foggy day. If someone catches up to you on the road, they will most likely see your car sooner w/ the rear fogs, especially if your car is a bland color (like my silver blue). I drive in these conditions quite frequently, which is why I insist on having working rear fog lights on all of my cars, even my domestic rigs. I've been rear-ended enough in the fog as it is.

We all know how much harder it has been to find decent w123s lately, I just want to protect what I've got. I figured I'd share the info with others, in case someone else is interested in doing the same. I'm hoping that someone wanting to add rear fog lights is quite aware of how to properly use them.

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Darren Asuncion
'81 300D Silver Blue metallic
'04 MINI Cooper S, Indi Blue w/ White roof
'94 Jeep Cherokee Sport, Flame Red
'95 Land Rover Discovery, Corallin Red

I did the same mod, however here's how I did it ... much easier

1. I modified the bulb holder to have 2 contacts on for the bulb
I did this by taking an extra bulb holder cutting off the tab that holds the copper contact and jb welding it to the side of my bulb holder ... now I can put a duel filament bulb in ...

2. I soldered a wire from the brake light socket to the new socket (the one for the bright filament .... I then soldered another wire from the fog light (running light ... the red one) to the dim filiment of my new socket ...

this gives me BOTH rear fog lights and 2 sets of brake lights and I didn't have to rip apart my interior sorry I didn't take pictures

Hmm. Sounds to me like you added a 2nd set of brake lights, along with an additional set of rear marker lights.

Rear fog lights are a bit different than marker (aka running) lights. They are roughly the same intensity as brake lights, and are only to be used in foggy situations. It doesn't sound like your setup utilizes the rear fog light switch in the dash.

If I have misunderstood what you posted, please let me know...

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Darren Asuncion
'81 300D Silver Blue metallic
'04 MINI Cooper S, Indi Blue w/ White roof
'94 Jeep Cherokee Sport, Flame Red
'95 Land Rover Discovery, Corallin Red

How come the 123 TDT's (wagons) didn't have rear fogs?

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)

The Euro w123 wagons had a rear fog light, but it was mounted to the bumper like the backup light. I have no idea why they didn't just (somehow) integrate those two with the tail light housings.

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Darren Asuncion
'81 300D Silver Blue metallic
'04 MINI Cooper S, Indi Blue w/ White roof
'94 Jeep Cherokee Sport, Flame Red
'95 Land Rover Discovery, Corallin Red