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Audio Amplifier Installation

1989 Mercedes-Benz 300E W124 Chassis

by Chris (Zeus)


The audio upgrade in my 1989 300E has been an ongoing process. I've replaced the stock Becker head unit with an Alpine unit, added a 6-disc Alpine Ai-NET changer in the glove box (perfect fit, btw) and re-wired the stock speakers.

Most recently, I've added a 4-channel Alpine V-power amplifier, model MRP-F240. I'll just deal with the amp install in this DIY and not go into choosing an amp or hooking up subs, etc. I chose the 4-channel amp to power the 4 existing stock speakers. I plan to replace the stock speakers with similar sized ones and mount a sub with another amp under the first aid kit area. But that's down the's how I installed my amp.

NOTE: I'm am amateur enthusiast, not a 'pro', so all advice is at your own risk, etc.


Tools I Used:

Materials Purchased:


I did some reading and planning before I jumped into installing the amp as this was the first time I'd tried installing a car amp. It's a good idea to have a basic routing layout in mind before starting. This install runs the power wires down the passenger side, the signal wires down the driver's side, and the amp is mounted in the trunk. Running the power wires down one side of the car and the signal wires on the other side reduces the chance of introducing noise into your audio signal. Make sure you have wires long enough to run the length of your car. The PG kit I bought was perfect.

Remove all floor mats and objects from front and back seats, as you'll be removing the back seats and working around the front floor area. Clean out and empty the trunk completely. Remove the trunk mat.

First thing I did was to run the power wire. I used a red, 8-gauge wire. The power wire also came with an in-line fuse holder and fuse, as well as connectors for the battery and amp terminals, which was handy. I crimped and soldered the connectors, as well as covered the soldered joint with heat-shrink tubing.

Raise the hood and disconnect the battery. Directly behind the battery, there is a black plastic shield that protects some important looking, modular computer component things. Remove this shield. Behind it, you will find a rubber grommet that allows access through the firewall. On mine, there was an unused rubber nipple on the grommet. I trimmed the nipple off and fed the red 8-gauge power wire through the grommet.

Feed the wire through a bit and you'll be able to access it from inside the car, on the passenger side. To access the wire, remove the inner lining of the glove box. There are little grey discs that come apart and can be gently pried out to remove the lining.
Remove the glove box light and door catch as well. Pull the liner out. You should be able to see the power wire coming through at this point, in the upper left.
Next, remove the lower bolster panel, directly below the glove box. There is a single Philips screw to remove, then the panel just pries out.
Remove the sidewall panel. It pries off. Once this panel is removed, you can route the wire though into the passenger seat area.
I removed all the floor mats and the floor cover from this side of the car as well.

While you are here, start running another wire alongside the power wire. This will be your amp shut-off wire. If you bought a kit, it will be the thin, blue wire on its own. Start it in the glove box area and then it simply runs along with the power wire. Leave a few feet of slack blue wire in the glove box as you will later run this end of the wire down to your head unit.

Pull the power wire all the way through into the cabin, leaving about 12 inches or so left in the engine compartment to hook up to the battery. Next, pry off the front door tread guard to expose the carpet edge. The tread guard is held onto the chassis by 3 metal snaps, it pries off upwards.
Using the pliers, gently pry off the little white tabs holding the carpet in place.
If carefully removed, these white tabs can be re-used. Run the power wire and shut-off wire under the carpet and along the sidewall.

Remove the lower center pillar cover by removing the two Philips screws that hold it in place. The cover then slides down and out.
Remove the rear door tread guard and white carpet tabs as well. Run the wires under all the carpet to the back seat area. Remove the lower back seat bench by depressing the red tabs at the lower edge of the bench. There are two tabs. Depress the tabs and lift the bench up and out. It's large and awkward but not heavy.

Remove the upper portion of the back seat by first unscrewing two Philips screws anchoring the seat. There is one on each side.
They require an 8 or 10mm socket (can't remember which). Once you've removed both screws, the seat lifts UPWARDS first, then out.
Pry back the backing material on the far passenger side, just beside the seat belt area and you will find another firewall access grommet.
Just push some wire through the grommet then open the trunk.

You'll now have a clean path to route the wires under all the carpet, along the sidewall, up and under the seats, under the backing material and through this grommet into the trunk cavity. I used some tape to hold the wires in place until I put the seats and carpet back in place. You want to ensure the wires don't get crimped or crushed against any metal edges or otherwise get damaged.

Remove the back cover inside the trunk to expose the gas tank and you should see the wire poking through.
For reference, the lower bolt in this picture beside the red power wire is the shock mount and the other bolt, on the black steel of the gas tank is the bolt I later used for grounding the amp. Now you've run your power and your shut off wires.

On the other side of the car, I ran the signal and speaker wires. This consisted of two sets of RCA cables and two sets of speaker wire. There is a matching access grommet on this side as well. Feed some of the wire through this grommet and leave enough slack wire in the trunk space to reach the amp.

Run the rest of the signal wires through into the trunk and I also ran two sets of speaker wire for connecting the front speakers. From inside the trunk cavity, you can access the bottoms of the existing rear factory speakers. I ran new speaker wire from these and left enough slack to reach the amp.
This picture shows the power wire, shut-off wire, RCA cables and new speaker wires.

A good tip I'd add at this point is to label one set of each of the RCA and speaker wires with a little bit of tape or by using a Sharpie marker. This is in case your wires are all identical - you will want to know which set is for the left side and which is for the right. I marked the left speaker wires and the left RCA cables with some electrical tape, on each end of the wires.

Run all of the signal and speaker wires down the driver's side of the car using the same procedure as the passenger side - remove door trim, carpet clips, pillar cover, etc.
I bundled the wires together with some plastic ties to keep them together. I then ran the wires underneath the driver's floor mat and across to the center console. From the center console, I ran the wires underneath the carpet and into the console through an access hole near the air vent on the console just underneath the ignition switch.
You will have to remove the lower bolster cover from this area as well (as per the passenger's side). This is only one routing option. Due to the restricted space above the driver's side knee bolster and around the steering column, I didn't want the wires anywhere near the steering components. This is why I elected to run the wires across the driver's floor mat to the console (near the shifter area) and then up under the carpet and into the console near the air vent. With the floor mats back in place, the wires are not visible.

The next bit was a little tricky.

I needed to feed all the rather bulky speaker and RCA signal wires through the small opening near the air vent into the cavity inside the console where the head unit is mounted.
This picture shows a view through the head unit mounting bracket (head unit removed) and into the cavity. The signal wires enter in the lower left area - near the white piece of tape around the thick black wire in the picture. It is tricky pushing the wires through into this cavity and retrieving them. I recommend a break at this point, as much cursing may well ensue.

Head inside, grab a pint. Drink beer. Breathe deeply. Listen to Pink Floyd's “High Hopes". Finish drinking your pint. Back to the install…

I taped together all the signal wires to make a single bundle with a tapered lead.
I staggered the RCA tips so they wouldn't bunch up. This was to eliminate my having to run every wire separately into the cavity. Again, this is only my personal amateurish procedure. I'm sure the pros do it differently, but this worked for me. I fed the tip through and was able to easily pull the whole bundle through and into the cavity.
This picture shows the wires through and ready to be connected to the head unit.

For reference - the green and black wires pictured here (in FIG. 23) are the leads to the front stock speakers. A handy test you can do to find out which set of wires is powering which speaker is to connect a 9V battery to the speaker wires. Simply attach the green wire to one terminal on the 9V battery and the black wire to the other terminal (doesn't matter which one) and you'll hear a crackling sound coming from the speaker that set of wires is connected to.

Rather than run new wire all the way to the front speaker terminals, I simply connected the speaker wires than I had run from the amp to the front speaker wires here in the cavity, using heat shrink tubing.

Back to the other side briefly, I then ran the blue shut-off wire from the glove box into the cavity. This is an easier run than the other side. Just fiddle with the wire a bit and you'll see it coming through in the upper right hand corner of the cavity. I had previously ran my Ai-NET cable through here, so I just followed it.

I connected everything up to the head unit - make sure you double-check that your leads are connected properly - and then I popped the head unit back in.
Here's the head unit installed again. I have an older Alpine unit, but I like it as this unit has dual 4V pre-outs, which is a nice feature (an expensive feature on newer units - most come with the lower 2V pre-outs).

Replace all the carpeting, panels and glove compartment liner. Make sure that the wires aren't crimped against any sharp metal and that they are safely tucked away as you replace the carpeting. Tie the wires together with plastic ties to make sure they stay out of the way.

That takes care of the front end, so back to the trunk to finish things up there. I connected all the wires on the amp and double-checked everything. The ground connection is critical to getting a nice, clean signal. I connected the ground wire to the chassis at the bolt shown
- the bolt on the gas tank mounting. I used an O-ring connector on one end of the ground wire, undid the gas tank mounting bolt, slipped the O-ring on, replaced the bolt and tightened it up snugly. I installed the fuse at the battery end and I connected the power wire to the positive battery terminal. I checked everything one last time and then I connected the negative battery lead and powered things up.


Here's the amp connected and powered up. I had one set of speaker wires mixed up - ok, a partial success then - but after I switched those, the system worked perfectly. The amp is very quiet and there is no background hiss or hum. I'm very pleased with the improvement. I haven't mounted the amp permanently yet as I plan to add another amp and sub.

So, next up will be an 8" or 10" sub mounted directly to the rear shelf underneath the first aid kit area. I'll add a second, smaller Alpine amp to power it. My current amp has a set of RCA pre-outs, so I'll simply run the RCAs from the existing amp to the sub amp and I'll enable the Hi-Pass filters on my 4-channel amp and the Lo-Pass filters on the sub amp and my system should be complete.

I'm not looking to win any comps or rattle my neighbor's windows, I just want a decent sounding system. So far so good! Adding this amp further improved the system's sound quality yet another notch. The biggest difference I notice is the improved bass from the stock speakers (albeit puny as it is) and the additional headroom before distortion. Once I add the sub/amp and set the Hi/Lo pass filters, the system should be good to go. I'll also replace the stock 4" speakers.

Any comments or questions, just PM me.

P.S. This install reminded me all too clearly that I need to clean and detail the interior of my car!


"Zeus" on the Shop Forum

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