I had the same problem on my 93. It turned out that the gray plastic piece at the end of the cable snapped off and I couldn't seem to glue it back together with anything--even PC-7.

I checked with the dealer and they said they only sell the whole regulator as a unit, which I think was in the $400 range. I hardly ever use the back windows anyway, so I took a piece of 3/4" PVC pipe and wedged it under the window with the pipe slightly bowed. I cut a 1/4" slot in the top of the PVC so the bottom of the window could rest and not shake loose. If I recall, I might have also safety wired the pipe so it wouldn't move.

That was a number of years ago and I haven't even thought about it until reading your post. If you go this route make sure you disconnect the power to the motor.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles