David,
the sunroofs with the motor in the front of the car (like in your C class) are only used since the later models (W202, W210). The older models, as the W126 S class, had the motor in the trunk.

I think that your sunroof has something broken in the tilt system (as constellation wrote). It is something that happens rather often on the older sunroof systems.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)