This seems to be a common problem on the dodge durango. The problem is located within the heater valve which opens and closes the heater core from allowing engine antifreeze from circulating through. The trick lies in whether or not the valve is electronically controlled or manually. If automatic, you'll have to check to see if the control pannal is bad or the valve on the heater core, if manual, it would only be the valve. Unfortunately this valve is not easily accessible, usually lying somewhere close to the heater core itself. However, there is a rather simple test to know for sure.
1) locate heater hoses (2 running side by side) from the engine bay area. (typically a black rubber hose, 1"-1.5" in diameter, usually running from the thermostat towards the firewall of the vehicle on the passenger side.
2) feel the hoses for warmth. (Note: car must be warmed up, (water temperature gauge normally half way up the display: typically around 160-180 degrees)
3) if warm, you know the problem is isolated somewhere between the heater core and the heater valve (valve is located at the end of both hoses which restricts them from circulating.
4) if heater core feels warm, then it is either too plugged up to allow air to flow through or the fan isnt working properly.
5) if heater core is not warm, the problem is the valve.
-if the heater hoses coming from the engine arent warm:
1) check to see if there is a valve on the end of the hoses (on the engine side) that is shut
2) the hoses are plugged off at the engine, or engine water temperature isnt running at normal temperatures.