It's a vacuum problem, the flaps that close the path from the heater core to the evaporator are not working. The side vents are on a separate system from the center vents. The problem most likely is a vacuum leak in a line or the climate control panel ( the buttons).
knutz

__________________
'90 190e 2.6 140K
By the time I got enough money to have fun and buy some toys , my body was to old and beyond repair !!!