Mine is a '99. Yes the Radiator is good as is the thermostat. Yes the pump is good & the coolant full. The heater core is new & grounded to prevent electrolysis that those engineers cleverly engineered into the system. A number of google searches revealed little understanding in hundreds of posts over many web sites. The blend door??? Please. Not my first trip to the Rodeo.
Near as I can tell, cold air is being routed someplace to dilute the heat and unfortunately my Haynes book is a sad excuse of a guide. Seems like cables were proven technology but perhaps too reliable. Decent truck otherwise. (except for the Sun gears)
Who here really understand these systems.

 

Are both heater hoses hot to the touch?