How can I locate the blend door for the heater/defroster

Suspect a defective blend door actuator motor that controls airflow for the defrost. This electric motor may be defective and in some makes and models can be extremely difficult/expensive laborwise to replace

Vacuum controls this circuit and leaks will cause blend air door to default to defrost. Check for bad check valve or hose leaks under hood first as these are the easy ones to get at and under hood heat causes deterioration.

Usually a sign of a faulty blend door under the dash.
The blend doors direct air thru the heater core and the desired vents.

If both heater hoses are hot, the problem could be a stuck or faulty blend door under the dash.
The blend doors direct air thru the heater core and out the vents.

For some reason this question was listed a dozen times within an hour or so.
Usually the problem is a blend door under the dash. The blend doors direct air thru the heater core and the vents.

Possible blend air door malfunction.

Usually a sign the blend door is not working under the dash. The blend doors direct air thru the vents and the heater core.

Usually this is a problem with the blend doors under the dash. You either have a vac leak, a dead motor, or a faulty door.
The control opens and closes the doors to direct air thru the heater core and evaporator core along with directing air thru the vents.
When a problem occurs, the default position is the defrost vents to make sure the windshield remains clear.
You can do an internet search for "repair durango blend door" for more info.

If you have the heater directed to the windshield, the AC will turn on. Move the control away from defrost.

Well FOR THE FIRST PROBLEM YOU PROBABLY NEED TO REPLACE THE THERMOSTAT IT IS MORE THAN LIKELY CLOSED SHUT NOT LETTING COOLANT INTO THE BLOCK TO CIRCULATE THE HEATED COOLANT 2ND THE UNIT IN WHICH CONTROLS THE TEMPERATURE OF THE INSIDE OF THE VEHICLE IS OR MAY BE BAD I WOULD START WITH THE THERMOSTAT FIRST BEING THIS IS USUALLY A MORE COST AFFECTIVE SOLUTION GOOD LUCK!