Right rear on my 300E has the same problem so last night I decided to tackle it. After chilling a six pack and consulting the MB 124 Body and Chassis Manual, I learned that the window was designed to be removed in the "open" position. Mine was closed! After a struggle and two beers I was able to push the unit away from the inner door enough to get a 19mm wrench on the two "shock bolts" that attach the bracket to the motor. This allowed enough slack to move it a little. I managed to remove the retaining clip from the bottom of the slide on the window and after another beer, wiggled the detached motor and bracket out of the razor lined slots with only a slight loss of skin. The saying about "Fools rush in where Angels fear to tread" was written referring to trying to repair the motor and regulator. My gears were worn and cruddy and after two hours and two more beers I still had a unit that didn't work properly. Take G-Benz' advise and snip the cable and order a new one. One point worth mentioning, be very careful when you pop the cover off the switch connector. There are six wires in there that spring out.
Fortunately there is a color code in there to show where they all go. Good Luck!! In case you're wondering, that last beer was consumed sitting on my roller stool thinking about what a great teacher experience is!!