I had this problem as heat was actually bellowing out towards the feet area. Great in winter but sucked in Summer !!

Turns out the AC amplifier needed replacement.

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Mark Herzig
1995 E300D 153K
1985 300D 142K (sold)
1979 450SL 122 miles (sold)
1992 500E 127K (sold)
1987 300SDL 132K (sold)
1986 300E 161K (sold)