Surging power windows, one won't close.

[BoostnBenz]

My 83 300DT was rear ended and among some of the weird things which happened in that instant was that the rear window can no longer have enough gear to make it to the top. I haven't looked at schematics yet, but I think the window motor may have moved on it which is causing it to not make it all the way. Trully what concerns me the most is that in both my 300s the rear windows kinda surge or jump all the way down. Is this common? What is causing it and how would one fix this? Is it foreshadowing the motor failure?

Thanks in advance,
Jeff M.