Okay, I just checked the manual. It does not mention anything about depressing multiple buttons simultaneously, positive or negative. So I'm not sure where I got the idea to do that.

I know that doing this is the only acceptable way I get heat to the floor. I used to press just the defrost and my feet would freeze. Or I would push just the middle button and my feet would be warm but I would get could air out the defrost.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD