I had my 1993 400E for a few years before I realized that the auto position would eventually regulate the blower motor speed somewhere between high and low once the temperature in the car reached what was set on the thermostat wheel.

Having said that, I have also replaced two blower motor regulators--both after they stopped blowing at all in low and very little in high.

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