I had the same problem on the right front window of my 87 300e. I had pulled the door panel and unbolted the unit, and pushed and pulled every which way, with no success. Then, I called my indy (see forum "good repair shops-New York"). He said before I come over, I should have someone push the console switch down while I simultaneously hit the motor lightly with a hammer. Although I snickered to myself when he said this, I tried it. It worked!! The window went down, and now it continues to operate. I have to lube the tracks so it can move smoothly-the plastic bits are all fine. Others have posted here that the window motors usually are not the culprit. So, if my window again quits, hopefully this technique will allow me to lower it for easy removal and replacement. Otherwise the unit apparently has to be cut because the motor locks the regulator jaws, making it immobile. Hope this helps.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)