I had similar problems with the blower coming on whenever it felt like it.
The switch was the problem. Make sure you have voltage at both fuse terminals of the blower motor while the car is running and the blower is on the on position but not working, that can be an indication that the blower is not getting any juice in the acc. position.
Hope that helps.
1993 400SEL (W140) 281K
1999 S500 (W140) 163K (Sold)
2002 S500 (W220) 72K
1999 S500 (W140) 88K
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