Noticed on my way home form Vacation today. My heat works fine. When I switched over to defrost, it is cool at best. It starts off warm very briefly, then cool air is all it blows. As soon as I switch to heat though, it's fine. I also noticed my heat guage seemed to be a bit low and my radiator cap was not even remotely warm, the upper radiator hose was too hot to handle though. It was cold today, 25-30 degrees according to the Truck, but don't belive it was like this on my way to vacation, with colder temps to. I did fill the radiator up with fluid before leaving, but don't see how that could effect things...


Truck is a 99 Durango, 360, slt+

 

T-stat may be stuck open and maybe a door in the ducting to your defroster is stuck?