Sounds like your push button control unit has gone South. Lot's of info in the archives on this topic. Over time a solder joint or two goes bad on the PBC circuit board -- if you're OK with soldering you can pull it and try to spot the bad joint or re-solder all the main joints. You can also buy a remanufactured unit ... try Fastlane.

You can test the output values of the PBC at a 3 pole connector right next to the brake booster. I think the colors are yellow, red and black. If you get the same resistance value for all 3 wires (high fan, low, off) then you know the PBC is shot.

If your blower motor, regulator and fuse are all OK I'd wager that this is exactly your problem.

Good Luck

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-Paul-
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi