The switch could have failed in the "up" position.

I have replaced two window switches for precisely that reason -- and a third just broke a few days ago. I tried taking them apart before, but I could not get them back together as they were originally.

As Haasman said, they never wear out; junk gets in there that chips and breaks the ends of the very thin plastic tubes that hold the springs, and which the ball bearings pivot on.

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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