*Low Coolant Level
*Air lock in the heater core
*Plugged heater core
*Stuck/sticking/fauly blend door or heater control valve (coolant)
*Thermostat


First thing to do is make sure the cooling system is full. Put the truck on an incline and let it warm up with the heater on full the entire time..running it without the cap may help as well (just be ready to put it on when the thermostat opens up). You may find some air works out and you gain your heat back. That is a very common occurance in german vehicles atleast.