car stereo install in 1992 mercedes

yea just to clarify, I'm not totally sure that harness is made by Metra.  I just assumed it was because Crutchfield sells only metra stuff I think.  and plus it has the crutchfield item number that ends in what looks like a metra part number.

What do your plugs look like?  I coulda sworn that that harness was the correct one but maybe I was thinking of the 90 and 91 E class.  Anyways, with the fader knob there's usually just 4 speaker wires  up at the radio's wiring :a positive and negative for the left side, and a positive and negative for the right side.  The rear speakers are wired into the fader (don't ask me how it works cuz I've tried to figure out before but never did find out--I just know that it works).

All you have to do is hook up either the 4 wires from the front speaker outputs (grey, grey/black, white, WHITE/ black)  OR the 4 wires from the rear speaker outputs (green, GREEN/ black, purple, PURPLE / black) of your aftermarket head unit to those vehicle's 4 speaker wires, and with the fader knob set at the middle position it should be 50/50 as would be expected.

Most likely unless you can get a photo of your wires for us to look at, you're gonna have to use a multimeter and speaker popper to determine the correct wires.  Although you really should be using a meter to verify the wires anyways.