It is inside the HCAV housing under the dash.
Either your thermostat has failed (engine stays cold) or your flap that isolates the airflow through your heater core has failed.
A Dodge Durango does not have a water control valve in the heater plumbing. The cabin temperature is controlled with airflow control doors.
99 Durango doesn't have a Heater Control Valve, It only has a thermostat
the heater core in the heater unit may not have air going through it, or the hoses for the water to pass through the heater core may be broken. Or the flaps controlling the airflow may not be working correctly
Its a Durango.......not a Drango.
How to replace the inside Spectra Heater Core on a 2000 dodge durango??
You may want to check your model year. There is no such thing as a 1990 Durango.
The heater output temperature is regulated by airflow control doors inside the dash, not a water valve.
If it has rear heat it has two heater cores. One in the front and one in the back.
A 2000 Chrysler 300m does not have a heater valve. The cabin temperature is controlled by airflow doors.
There isn't a water control valve.
The most likely is the vacuum line used to operate the swinging doors to redirect airflow is broken or not in place.