I recommend abandoning the existing wiring and running all new wires. In the 123 body it's fairly easy to do, just lift up the carpets. I replaced the speakers with MB-Quart speakers, there's a thread here somewhere on that project. You can abandon the fader switch in the console and use the controls in the radio so that you have true 4-speaker stereo. I ended up installing a switch in the fader position and wiring it to my garage door opener remote (there's a thread on that, too).

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970