92 318is heating

Over the last couple of months my heater will only blow hot air out of the windscreen and foot vents, but will happily blow cold out of the
center vents. I have had no choice but to not have the blower on at all. This leads to the centre console becoming very hot. I know it will default to full heating if some component fails but do not know where to start in diagnosing the problem. It has manual heating control and not climate control. Any help would be greatly appreciated. Cheers Matthew.
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