Old.partially dried lube (On the Rails and in the cable tube and gearbox)= friction
Typically lack of "Servicing" the unit leads to Motor demise (It burns up)
You may have more than one "Binding" point created by the No-Lube Issue.
The only Lube to use is "Gleitpaste" [does not attract dust]
Your should clean and re-lube your Rails whenever you change your oil.
(Hardly anyone does!)
Your unit may have a friction type clutch (Adjustable).
BUT ,If everything is not clean and lubed properly tightening the clutch can
take out both the clutch and the motor.


You may need new gearset (If that model S'roof has any)

I've no experience with your model's S'roof.

If it were a 123,124 or 126 I'd suggest taking down the S'roof headliner to
be able to unbolt the cable from the front of the S'roof.
So you could slide it open manually to be able to disassemble/clean.

__________________
'84 300SD sold
124.128