126 Climate Control

[fintail]

OK, first off I know there are many threads about 126 climate control issues. I did read through them, but I can't find one with a problem exactly like mine.

Like in many threads I read, the heat in my 126 (300SE 1989) doesn't work correctly. I get warm (not hot) air out of the side vents, but cold/ambient air from the center vents, no matter the temp. wheel position. All fan speed setting work correctly. I also get warm air in the footwells, after several minutes. The only way for me to warm up the car is to run defrost to get the warm air circulating from the windshield, or to drive for a while until the warm footwell air circulates. Does this sound like a monovalve problem as well, or could it be some weird vacuum thing or a heater flap, as the center vents seem to be the problem?

BTW, the AC in this car works well, if it matters.