This happens on the W126 coupes with some frequency--the anchors for the window mechanism are insufficiently bolstered for the thin sheetmetal door skin and it tears. Some epoxies might work rather than risk welding. Reference:
http://db.mercedes.cx/sec/200407/26/0040.html
http://db.mercedes.cx/sec/200301/26/0000.html

Cheers, Andrew Seidel

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86 560SEC
85 500SEC Lorinser
83 300D Turbo
75 240D