Just a quick thought ... do you see any evidence of corrosion or general weirdness in the glass? Especially around where the power attaches to the window?

I had a rear window in a 124 series that had that and due to a leaking rear window gasket it allowed water to corrode the lead and it broke off. Needed to replace the window ($).

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)