I've only seen mine, but it was caused by a bushing failure misaligning the regulator. I can't get my shop manual cd to work, but if i recall correctly, you just wedge the glass up. Would be a good time to change it though. My motors are still fine in the '89. Just make sure to replace the moisture barriers. While you are in there, it's a good time to change the door handle if yours is drooping--not too expensive.

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Earl McLain
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)