The switch unit in the drivers door contains the "brains" and will accumulate water if it gets rained on. Newer versions have a drain hole. I know, because mine went bad (no up or down) and I opened it up and found the bottom inch of the electronics all corroded away (much like the tail lights...). So, consider ordering a switch module from eBay to help with troubleshooting. I think new ones are around $100. Haven't checked eBay in a while.

Other people have reported the spontaneous window opening problem. One that I recall left his car at the airport for a couple of weeks and came back to find it really soggy....

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles