I just replaced the regulator assembly on my 88 300CE - the drivers side front window. I know its different but..

The whole assembly - motor and regulator was $100 from potomac( an MB junkyard) - they are in Maryland but will ship anywhere - I'm in PA.

It took me about an hour to install it - and thats because I was in no hurry.

I wouldn't try to rebuild it or buy it new. Its the rear window and probably gets less use than the front ones.