Internal heating thermostat problem

I have a 1992 E36 320i with the independant heater controls (passenger and driver). I can regulate the heat going to the vents on the passenmger side
but am only getting warm air from vents on the drivers side even when turning the heater control to the minimum. This also happens on the defrost. If I turn both driver and passenger heater controls to minimum, I get warm air from the defrost vents on the drivers side and cool air from vents on the passneger side.
Mystifying???
All help/advice welcome.
Thanks
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