Well, the real problem is that you keep on thinking about that totally useless excuse for an automobile known as the Q45
Most non microfiltered/early E34s have a mouse nest or two in the heater box (the theory here is that ones with microfilters get cleaned more often than not) which can stink up the air (egh!) and minimize air thoroughput. So, check for "backup behind [sic: actually, in front of] the evaporator."
The heat discrepancy could be a sticking heater valve, flap issue (unlikely) or potentiometer issue in the vent temp wheel (also unlikely since that pot regulates both sides). Searching "heater valve" will return myriad results, so do that first.
best, whit