I just went through this on my E420 -- It was the blower motor. Last year, when I changed the regulator, I also oiled the blower because it was squeaking. I went through four strip fuses and found that it would only melt the fuse when it was in auto or high. DO NOT replace it with a URO motor. Get the Bosch or Behr. You will save yourself the hassle of doing the job twice.

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