Heat in cabin does not get hot enough in cold weather

I have a 93 190E and the heat in the cabin does not get hot enough. Would the Climate Control Unit with its potentiometer be the problem?
There is interior temp sensor (At the dome light) and a temp sensor on the outside of the heat exchanger. I have flushed the heater core, changed the auxiliary water pump and the heater valve and the problem still exists.
Does anyone have experience with this problem???
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