The cheap plastic part just holds the window to the control arm. If this part fails your window will dismount from the control arm and will be stucked at the bottom of the door.

What you described on your 95 S 420 looks more like a control arm failure (very common failure on W140s, especially on rear windows) and/or window motor failure (due to an extended unused period of time).

Any of these two case scenarios would be expensive to fix ($$$), unlike the cheap plastic part described originally on this post.

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A. Rosich
CL 500, 1998
S 500 L, 1998
E 320 T, 1995 [Sadly sold ]