This happened to me twice and both times it was the cable that snapped after getting tangled around the plastic regulator wheels. (The plastic slides were fine.)

You should really be looking at whole regulators (without motors.) It is a quick, easy repair.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black