Rear window regulators on the w124 are very fragile.

They use a cable drive and the plastic piece the window attaches to, and is riding on the vertical track, is often broken. check to ensure the plastic is still clipped onto the vertical track.

The same vertical tack can easily be tweaked, and that will also cause the window to not operate properly.

There is a clip at the top of the assembly that requires the window to be down slightly to release, of you can bend the donor door inside panel to get it out.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles