Heat doesn't come out of the center vents. Heat comes out of the side vents, the lower legroom vents and the defroster vents (when you push defrost, otherwise a little bit of heat comes out of the defrost vents, "enough to keep the windshield clear")

A/C comes out of the center vents. (A/C also comes out of the side vents)
(According to the Service Manual, Heating, Air Conditioning, Automatic Climate Control W123)
(I was looking at the section on Climate Control III like in my 1985 300DT, haven't looked at the older systems yet.)