Sounds like a broken window regulator. I just replaced one on my 80
300SD yesterday. It's a big help to have a service manual, so you know
how it all comes apart. The good news for me, at least, was I got an
aftermarket one on Ebay for $70.
Only problems I had were :
1 - One of the threaded holes in the replacement one were not cut
correctly and after getting it into position inside the door and
starting the other bolts, I had to tap the damn hole. Fortunately, I
had the 6mm tap. And I know why. I remember having to do this on the
last one I bought about 6 years ago.
2 - After getting the whole damn thing in, I went to adjust the stop
that limits how far down it can go. There is a screw and stop on the
window regulator. And to get to it, there is a round hole on the
inside door sheet metal. Well guess, what. The old one had an allen
head screw, so the hole in the door is plenty big for that. But the
replacement one has a regular hex head bolt and the hole in the door
aint big enough for a socket! Boy was I pissed! Luckily I was
barely able to get the bolt out, replace it with the allen head one
from the old one, and do this without the damn stop falling off inside
the door! You;t think the guys that make these things would realize
this stuff, but apparently not. Still bettern that paying $$$$ for
one from Bavaria.
So, all in all, it went pretty well and she's running up and down nice