1991 BMW 520 Heating Problem

Heater matrix, what is that? The issue may be in the heater control valve at the firewall, drivers side. Could you also give more details, when did this start, did anything else happen, has anything been tried yet? Realoem. Com is a site that I use to get to part numbers and exploded views, go there and see if it is helpful to you, look inthe 64 section for heating and air conditioning.