Friends:

My AC has been fine, until a couple of weeks ago. Now, it seems when I slow down, hot air comes out of the driver's vent and the left most center vent, and cold air comes out of the passenger side vent and the right most center vent. This car (1983 300D) doesn't have, as far as I can tell, a left/right setting (as did my earlier 1979 240D). This also seems to happen once I reduce speed. What gives? What could this be due to?
Any help and advice is greatly appreciated, thanks so much in advance!

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1983 300D (Hans)