I get a light on my dashboard occasionally that a light is out. It's always the right rear side marker...... always.

I've previously scraped off any calcium buildup (or whatever that stuff is) on the metal bar that holds all the bulbs, I've changed the bulb, plus I've put dielectric grease on the bulb base. It works sometimes, but then I get a notification on the dash and..... it's the right rear bulb again.

I had my mechanic look at the car. The dash indicator light came on. I told him it's always this right rear side marker bulb. ....always. He looks and.... the bulb is working fine.

So he looks around the rest of the car and this time it was the FRONT LEFT marker this time! I replace the front left bulb a few days later, it's working fine, I start the car up.... the right rear bulb is not working again plus my left turn signal light stays on constantly (not blinking) on the dash.

What is up with this? How do I fix this?

1991 300d 214k

Earthing fault , check and clean all earthing points on light units and also engine gearbox earth strap.

Not sure what you mean by earthing fault or the engine gearbox earth strap. Where is this?

Thanks.

In the US we refer to "earth" as "ground". On a vehicle made since the mid 50s, the chassis and body are usually one big "ground" plane connected to battery negative.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Don't overlook the connector plug on the taillight assembly. The pins can get oxidized and cause similar problems. Been there, done that.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

If the earth side of connection is bad it will cause the weird symptoms you describe. Clean all the connections on lights and turn signals and any major earth connections you can find.
Earthing strap from engine is connected to gearbox and crosses over to the chassis ,should be on the drivers side underneath the car

Take a close look at how the bulb holder plate attaches to the tail light housing. Chances are the plastic tabs have deformed and aren't holding the ground tab against the bulb.

Sixto
98 E320s sedan and wagon

It can also be the N7 bulb monitor relay failing.

I had this problem with my right front marker filament in the turn signal. Fortunately I was able to open up the housing and find the cold solder joint.

This part is very common in the 124 and gen 2 126 so I always grab them when I am in the yard. It is the long relay in the back of the fuse box and has nearly 2 dozen pins so it's in there tight.

Are you getting voltage at the connector for the tail light assembly? If you are not getting voltage I would suspect the N7.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

My mechanic tested a bunch of stuff and thinks the tail lights are fine. The problem is the headlight switch on the dash! So I'll put a new one in and see what happens.

Not sure about that diagnosis. If the switch were at fault, you'd be losing the front corner marker as well. The power feed for it is common with rear on the same side.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Laugh as you may, but the original problem might have been a bad headlamp switch, but once I replaced it with a new one, I STILL had a light problem. Turns out I used the WRONG LIGHT BULB in the front side marker. It was a 12v bulb and it DID work when I put it in. The problem was that it made the other one NOT WORK RIGHT.

Why?

Because the CORRECT BULB has TWO connections on the bottom of it. The bulb that I put in only had ONE connector on the bottom of it, which was causing problems.

Problem FIXED!