As it so happens, I have the same age and model car. As it also happens, my sunroof does not work either. However, this was a combination of the plastic MB used to secure the sunroof to the cable and the age of the car. Upon inspection, the sunroof mechanisms that sit on either side of the sunroof were completely busted. I sourced a replacement at a junkyard. I have yet to install it however. It is my belief that the motor rarely goes and the same with the cable. However, the fact that MB used cheap plastic and cheap metal parts to form the mechanism would lead me to look there first if I suffered this problem again.

Can you physically open the sunroof (ie push it up from the inside?)? Another way to check the motor would be to break out the little too MB shipped w/ your car or just a wrench and try to open the sunroof/close it manually.

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87 300E
202k on the clock