It's probably the plastic piece that slides in the bottom of the window track. One broke in my 300sdl last month. I bought the whole window motor assembly from a junkyard. It turns out, you can buy the plastic piece from fastlane. You have to remove the motor mechanism and grind off the old rivet. Install the new part and pound the rivet down then reinstall the motor assembly. The part is called a window regulator kit.

Take your door panel off and you should be able to tell if that's the failed part.

Dave

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1986 300SDL