I've recently replaced the heater matrix and noticed this after fitting it
found the cables which are fitted either side of the heater bix came out of their clips and not switching between hot and cold or shutting off the heater box flaps , which if not fitted correctly will allow cold and hot through the vents at the same time
i belive the ac comes on automatically when the car is first started onto the windscreen in cold weather, regardless itu have the ac switch off
see if the red and black cables are located firmly , if not u can get to them without removing the dashboard, learnt a few tricks in getting to them without too much trouble
after i done the cables the heater through all positions works really goodand can't have the switch above 2 as the car gets too hot
hope that may help abit