Bob, not sure if your setup is the same as mine, but happy to look at it for you.
My car has the heat/cool dial that is connected to a very long metal arm which then controls a circular wheel which opens or closes the heater tap. At one point, I had to tighten the screw on this wheel to get it to close properly (I think it must have come loose at some point) -so, at the moment, everything seems to work (finally!!)
Drop in some time and I can take a look.