Where is my heater control valve located?

Check the heater hoses to see if they are getting hot. Those S-trucks used Dexcool which tended to plug up the heater cores. If hoses are not hot, then haev the heater core backflushed to clear it out. If hot, then you probably havea blend door actuator problem or a bad control head

First, check to see if the temp door actuator moves when you turn the temp control knob. You should be able to see the motor shaft turn.
It is located behind the glovebox, on top of the HVAC box.
It is number 1 in the pic.

The control head is the panel with the vent/heater/ a/c controls