[Don Fox]

Hi RIchard:
What engine is in your Durango, and have you checked the coolant level? It can have enough to keep the engine cool in moderate temps, but not enough to circulate through the core(s) to provide heat.

Don

 

[Don Fox]

Ok on the 5.7:
Does it have the rear HVAC also? Do you see any puddles or smell coolant on a daily basis? Did you purge the air from the system properly after the nww T stat went in? If not, you probably have air in there.
The bigger issue though is the leakage. You'll want to find that asap.

Don