I had to lift up the window by hand. The plastic piece that slides along a channel at the base of the window had broken off. I could see the "arm" part moving when I pressed the window switch. It just had nothing to move against.
The matchbook was a suggestion by a local garage owner to keep the window up in case it rained before I could get it fixed right. (It broke on Friday night on a holiday weekend.) Luckily I already had an appointment set up for an oil change the following Tuesday and the shop was able to "R&R window regulator/motor assembly to replace slider jaw".
Cost $158 with the oil change, and I had them pull the Kleen-Wheels dust shields off my front wheels. Not sure if the 300E is identical, but if you can see your motor moving an arm but the window doesn't move, very likely the slider jaw broke.
Robert W. Roe
1984 300SD 165,505 miles