Same thing happened with my C220, and it was a plastic bracket that held the gears together. I took door cover off one time and saw the bracket which when broken didnt allow gears to contact and in turn move window. I called the dealer and they said they only sold the entire mechanism as a unit, at a cost of 400 +, so I went back glued the two pieces together put them back on track and then put some strong duct tape as well to keep pieces from splitting again in case glue gave out.

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1994 S320
131K miles

1998 ML 320
126K miles

1994 Blue C220
122K miles

Im going to Grad School @ the University of Iowa
Go Hawkeyes!!!