I haven't been into the mirror on my MB, but just finished with the power mirror on my Toyota. On the Toy, there are 4 small electric motors turning screws to raise, lower, move right, or move mirror left. The tech told me to hold the switch in each of the 4 positions, while he listened to the mirror. He heard motor noise in each of the 4 positions, and concluded it was not the switch or motors, but the screws. I later pulled the mirror apart, and saw that the plastic worm gear screws were stripped on 2 of the motors.
The MB mirror may be completely different, but listening for the motor noise in each position should be a good starting point. Even if there is only 1 motor in there, noise from each of the 4 positions would indicate the switch and motor(s) are working. No noise brings you back to square one.
At that point you could try a new or used switch (they plug in once the center wood is removed). Or if you are real handy try pulling the switch, making a series of jumpers, and reverse the right/left pins.
If the switch and motors are OK, it is the (unknown-to-me) mechanical drive between the motors and the mirror. Phil in Fastlane may know if mirror parts are available - they weren't on my Toyota. If no parts, a new or used mirror is in the forecast. Good luck.
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1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
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