There is a valve in the inlet heater hose or in the heater box that lets hot water in the box. it is probally not working, (leting hot water in the box). cant remeber if its elec or manuel on your truck. but its controlled by the hot to cold selector on your on your dash. OK LOOKED IT UP COULDNT REMEMBER THE NAME. Heater control valve looks like auto zone sales one for about $12. Thats probally the problem.