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| In my experience, there are three probable culprits. The head unit (the knobs you turn to adjust fan speed, temp etc...), the blend door actuator, and the blend door itself. It could also be wiring, but that is less likely.
The first thing I would suggest is locating the blend door actuator, and watching to see if it moves when you rotate the heat control knob from hot to cold, etc.. The little white part on the actuator rotates. If it does not move, its either the head unit or the blend door actuator. You could research what the voltage should be going to the actuator as you turn the heat knob, that would eliminate the actuator vs the head unit.
See attached picture for actuator locations with the dash off.
 (GMT400 HVAC (full).png)
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 GMT400 HVAC (full).png (165KB - 1 downloads)
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1998 Chevy Pickup heater sticks on hot
