When my regulator started to go, it would exhibit this kind of strange behavior too. Usually it didn't work in low. It would work sometimes in auto and sometimes in high. At other times it wouldn't work at all. I would start by taking out the strip fuse and checking it closely. Sometimes it can look good but it is actually cracked when you take it out.

I bought my replacement from Fastlane. I was always amazed at the engineering that went into the MB blower regulators. Too bad they couldn't have engineered them to last longer.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles