Stuck hex bolts are the reason I bought a dremel kit. Using the little cut-off disc, you can either make a slot and use a screwdriver to remove the bolt, or completely remove the head and deal with the stud. Hopefully, there would be enough stud sticking out for you to work with, and it would be easier to get at with the window motor disassembled.

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95 E320 Cabriolet, 169K