Why would the AC compressor come on with the heater on? I know it will come on with the defrost but this comes on in all vent positions? Im at a loss??

 

You know your 2000 truck is really a Ford Ranger, right?

The AC "can" run while the heater is on, how the factory designed it for cases where one was too cold.

Are you saying that the AC compressor is always engaged even though its separate AC on-off switch is NOT actuated? And that you cannot shut the compressor off?