Is this a factory A/C truck (even if it’s not working anymore). I will assume it is.
It’s very likely you have a bunch of junk dropped down inside the heater plenum, and maybe enough to block it. Take off the glove compartment door (screws go straight up into the hinge), and then remove the plastic plenum piece that is vertical right behind it. Scoop out whatever you find in there, and then move the heat control back and forth to make sure nothing is blocking the blend door movement. Look carefully inside and behind the door, because junk moves around in there. The heat blend door control is mechanical and it just adjusts how much air goes through the heater core, so it determines how much heat you get.
Now start up the truck and make sure any vacuum operated doors you can see are also moving. The vacuum-operated doors determine whether you get heat, defrost, A/C positions from the dash control. The vacuum for them comes from a round black canister under the hood, mounted near the blower motor. It’s fairly common for a shade tree mechanic to disconnect the vacuum lines under the hood as part of “simplifying all the underhood crap”, so you might not have vacuum to the controls.
Bruce
