I had similar problem with my 98 E320. Problem was blower motor/regulator.
I had to replace both regulator and motor. In some cases you may just need to replace the regulator.

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Donwo
1999 SL 500 Sport (30K miles)
1998 E320 (SOLD)
1989 560SL (SOLD)
1987 300E (TOTALED)
1986 300E (SOLD)
1984 300SD (SOLD)