check to see you have a thermostat installed. If you have one then you have a damper open due to no vaccum or corroded or foam seal needs replacing
The temperature blend door may be at fault.
Thermostat stuck in the open position. Replace the thermostat.
you may not have the heater water hoses plugged in
Cause its broke!
When you have the heater on, does it just blow any air at all? or no air?
You tow your offending 99 Durango to an empty field, fill the entire cabin with highly explosive and flamable materials and blow it to kingdom come where all the other piece of crap Durango should have been shipped straight from the factory! Worst piece of junk Dodge ever manufactured. I am ashamed to say that i was a die hard mopar girl until this monstrosity came into my life. I NOW DRIVE A FORD
Assuming you want it to blow hot, it could be low on coolant, a stuck open thermostat, a plugged heater core or a faulty temp blend door.
Leave it turned off.... doh
Because your thermostat is probably stuck open
Low Coolant? Bad thermostat? Heater core plugged or restricted? Heater core airbound? Heater control cable not working?
Check your thermostat first, if that's OK then your heater core is likely bad
The thermostat could be stuck open.