What are you refering to by "heater pot" ? I noticed in the Haynes manual they show 2 knobs for the heater control. Mine has 1 temp select knob and pushbuttons for the defrost/heater etc functions. There are 2 pots on the circuit board that that this knob controls. Is one of these the pot your refering to?

Thanks every one for your advice. I'll start checking it out this weekend and let you know what I find.