1980 300SD Speaker Wiring

My daughter bought a 1980 300SD. It's a nice car and we only know of one 'major' problem. The radio doesn't work! The previous owner said it hasn't
worked for some time.
No problem. She wanted to use her radio from her old car which has a MP3 player. So I started the exchange today. Here's the problem.
Apparently the speaker wiring is the problem. The speaker wires behind the radio appear to be unchanged (no splices and cuts). I performed a continuity check from these wires to the rear speakers and on the left. The speaker negative wire connection is good but the positive wire from that same speaker connects to the same wire under the dash!? The right rear speakers have a similar problem. The negative wire connects but the positive wire apparently is not connected.
My conclusion is someone messed with the speaker wires. A glance under the dash and in the trunk showed no visual flaws in the wire routing but there's not much to see without removing panels.
I have not done any experiments on the speakers in the door panels (I assume there are speakers there) because it was time to watch football. So before I go back to the task tomorrow (after the Cowboy game) and would appreciate any words of wisdom from you folks.
Are there any obvious places where the wiring is routed that I can check? Which side does it route? Is the door panel a pain in the neck to remove to access the front speakers? Or, should I just forget it and route new wires to the back and forget about trying to solve the existing problem?
Thanks in advance, Mark Austin, Texas
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Mark - I don't know about your car, specifically, but some radios are two pieces: a tuner head in the dash and an amplifier in the trunk. Pull the cover panels in the trunk and trace the speaker wiring.
Al

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I own one of these and can't tell you much about wire routing but can tell you that the original set up was for 4 speakers with a "fader" on the console.
Two speakers are in the corners of the dash and two are behind the back seat in the corners of the parcel tray. The "fader" distributed the sound between the front and back. There's no left vs. right adjustment.
The "fader" in the console is lousy in that it's crackly and has dead spots, so don't go out of your way to incorporate it into your new scheme.
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I believe you are correct, the fader is his problem. I replaced my radio in my 80 300SD long ago and eliminated the fader control, which I think, you pretty much have to do. The original Becker radio had the fader in the speaker circuit, the back one for sure, the front ones I think went direct. All new radios have the function in the unit itself now and I don't think are compatible with putting resistors in the speaker circuits.
The fader is a thumbwheel located on the right side of the center console, near the passenger side window switches. I removed mine, figured out the wires and connected the speaker wires direct. You could leave the thumbwheel in; I chose to replace mine with a cover piece. I got a piece of hobby plastic, with a small checkerd finish. Cut it to fit flush in the opening, spray painted it black, and it looks really great.
The other choice of course, if you want to retain the original radio for a classic car, would be to buy a working original one. There are places I've seen on the internet that specialize in these. Wish I had kept my original Becker, which was working just fine when I chucked it out. Could get quite a few bucks for one now.
To get decent sound, you should replace all the speakers, which will make a huge difference. There are some good small speakers for the front, though the space is very limited. For the back, I'd go with plate speakers, which have seperate woofer and tweeter. The finished speaker housing sits on top of the rear deck, with only the lower part of the woofer extending into the existing hole. You could also fit a 10" sub woofer into the rear deck, and a 340 watt amp in the trunk, like I did, and then she'd really rock.
Highest compliment I got on my mechanical skills was when I took the car into mercedes for service. When I picked it up, they wanted to know who did the stereo install, they said they couldn't get work done like that.
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