bmw 323ci airconditioning problem

The right side vents on my 2000 323ci blows warm air when the ac is on. I checked the freon charge and it is ok. I think that the heater is
partially on but only on the right side. I have been told there are two sides to the heater and a valve might be sticking. Does anyone know where the valve is located and how to access it or any other possible causes for this problem.
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