Rear air vent blowing hot air

That sounds like one of the blend actuator cams has failed. The method of testing it depends on which system you have.
Do you have the dual climate controls where the driver/passenger can select there own temperatures? If yes then you need a factory level scan tool to talk to the HVAC controller and use the tool to run the tests on the actuators and see if there are any stored codes in the system.
If you have the manual system then it could still be the actuator but the test procedure can be done from the control head.

First step will be to turn the key "on" and refer to the image attached. Turn the blower motor control ON (#1). Now push and hold the A/C button (#2) down and turn the blower switch to "off". Watch that the LED's (#3) all come on. If they do continue to the next step. If they do not try the test again. If they still do not light you may have a code 33 set or a faulty control head.

If they do light up the next step you perform the same steps but instead of holding the AC button down until the LED's light, you release it after three seconds. This will put the controller into diagnostic mode. If there are faults stored the LED in the A/C switch will start to flash (left side #3 in picture.) If there are no faults stored the LED's will turn off and the system will exit test mode.

If the light starts flashing you can read the codes by counting the number and duration of the flashes. The codes will be two numbers. Long pulses (almost a full second long) will mean the first digit and short pulses (under half a second) are the second digit. If you look at the second picture you will see how that works, but basically if you see two long pulses, then two short pulses it means 22, if it was one long and five short it would mean 15.

The system will continue in test mode and cycle through any codes, then the LED will stay on for two seconds, then go off for two seconds, and then it will start flashing the codes again from the beginning.

If you find stored codes you can post them here and we can tell you what they mean.

To exit the testing after you have read any stored codes, either turn off the key, or turn the blower motor speed to off.

Another thing you can try regardless of which system you have and if you find no codes (or even if you do) would be to disconnect the negative battery cable for a few minutes. Then reconnect it and see if the system cycles through its relearn and it starts working again. Be prepared though as this can cause the transmission and engine to forget things like shift pressures and idle speed until you drive the car a while, It will not hurt it but it can make it act strange until it relearns everything.

Now if you have done those tests or used a scan tool to test and still have no control over the center duct. The repair depends on which system you have because they use different control parts but other than a fault in the actual control head, the doors that control the ducts are all inside the HVAC unit. There are two actuators located where they can be replaced without an inordinate amount of work (however as you say you have good A/C in the front seats those appear to be operating okay) the rest of the system is all behind the dash which has to be removed to access the various cams and rods that make the system operate. That process is in the third picture if you wish to read up on it.

Hi and thanks for using 2CarPros. Com. There could be a few different issues causing this problem. I am going to start from under the dash and go through the system for you to check/diagnose.

Starting with the HVAC Housing Assembly: (see picture one)

All models are equipped with a common HVAC housing assembly that combines A/C and heating capabilities into a single unit mounted within the passenger compartment. The HVAC housing assembly consists of three separate housings:
HVAC housing - The HVAC housing (1) is mounted to the dash panel behind the instrument panel and contains the A/C evaporator and the blower motor resistor or power model, depending on application. The HVAC housing consists of a upper and a lower housing that are attached together and has mounting provisions for the air inlet housing, blower motor, air distribution housing and the HVAC wire harness.
Air inlet housing - The air inlet housing (2) is mounted to the right end of the HVAC housing and contains the recirculation-air door and actuator.
Air distribution housing - The air distribution housing (3) is mounted to the rear of the HVAC housing and contains the heater core, blend-air door(s) and actuator(s), mode-air doors and actuator and door linkage.
The heating-A/C system is a blend-air type system. The blend-air doors control the amount of conditioned air that is allowed to flow through, or around, the heater core. The dual zone heating A/C system uses two blend door actuators while the single zone system uses only one blend door actuator.

The A/C system is designed for the use of a non-CFC, R-134a refrigerant and uses an A/C evaporator to cool and dehumidify the incoming air prior to blending it with the heated air. A temperature control determines the discharge air temperature by operating the blend door actuator(s), which moves the blend-air door(s). This allows an almost immediate control of the output air temperature of the system. The mode door actuator operates the mode-air doors which direct the flow of the conditioned air out the various air outlets, depending on the mode selected. The recirculation door actuator operates the recirculation-air door which closes off the fresh air intake and recirculates the air already inside the vehicle. The electric door actuators are connected to the vehicle electrical system by the HVAC wire harness. The blower motor controls the velocity of air flowing through the HVAC housing assembly by spinning the blower wheel within the HVAC housing at the selected speed by use of the blower motor resistor or power model, depending on application.

The air distribution housing must be removed from the HVAC housing and disassembled for service of the mode-air and blend-air doors. The air inlet housing must be removed from HVAC housing and disassembled for service of the recirculation-air door. The HVAC housing must be removed from the vehicle and disassembled for service of the A/C evaporator.

Pictures 2, 3, 4, 5, and 6 are of the duct work to the rear of the vehicle.

If everything is properly attached, then I would suggest checking the HVAC mode door actuator. Picture 7 and 8 are of the mode actuator. See attached description:

The mode door actuator (1) is a reversible, 12-volt direct current (DC), servo motor. The mode door actuator is located on the driver side end of the HVAC air distribution housing, close to the instrument panel. The mode door actuator is mechanically connected to the floor, defrost/demist and the panel-air doors.

The mode door actuator is interchangeable with the actuators for the blend-air door(s) and the recirculation-air door. Each actuator is contained within an identical black molded plastic housing with an integral wire connector receptacle (2). Each actuator also has an identical output shaft with splines (3) that connects it to its door linkage and three integral mounting tabs (4) that allow the actuator to be secured to the HVAC housing. The mode door actuator does not require mechanical indexing to the mode-air doors, as it is electronically calibrated by the A/C-heater control.

The mode door actuator is connected to the A/C-heater control through the vehicle electrical system by a dedicated two-wire lead and connector of the HVAC wire harness. The mode door actuator can move the floor, defrost/demist and the panel-air doors in two directions. When the A/C-heater control pulls the voltage on one side of the motor connection high and the other connection low, the mode-air doors will move in one direction. When the A/C-heater control reverses the polarity of the voltage to the motor, the mode-air doors moves in the opposite direction.

When the A/C-heater control makes the voltage to both connections high or both connections low, the mode-air doors stop and will not move. The A/C-heater control uses a pulse-count positioning system to monitor the operation and relative position of the mode door actuator and the mode-air doors. The A/C-heater control learns the mode-air doors stop position during the calibration procedure and will store a diagnostic trouble code (DTC) for any problems it detects in the mode door actuator circuits.

The mode door actuator is diagnosed using a scan tool.

Finally, the blend air door actuator. It determines air temperature. See description:

The blend door actuators (1) are reversible, 12 volt direct current (DC), servo motors. Models equipped with the MTC single zone heating-A/C system have a single blend-air door, which is controlled by a single blend door actuator. Models with the ATC dual zone heating-A/C system have two blend-air doors, which are controlled by two blend door actuators.

The blend door actuator for the single zone heating-A/C system is located on the driver side end of the HVAC air distribution housing, close to the dash panel.

For the dual zone heating-A/C system, the same blend door actuator used for the single zone system becomes the driver side blend door actuator, which is mechanically connected to only the driver side blend-air door. A second separate blend door actuator is also located on the passenger side of the HVAC air distribution housing which is mechanically connected to only the passenger side blend-air door.

The blend door actuators are interchangeable with each other, as well as with the actuators for the mode-air door and the recirculation-air door. Each actuator is contained within an identical black molded plastic housing with an integral wire connector receptacle (2). Each actuator also has an identical output shaft with splines (3) that connects it to its respective door linkage and three integral mounting tabs (4) that allow the actuator to be secured to the HVAC housing. The blend door actuators do not require mechanical indexing to the blend-air doors, as they are electronically calibrated by the A/C-heater control.

The blend door actuators are connected to the A/C-heater control through the vehicle electrical system by a dedicated two-wire lead and connector of the HVAC wire harness. The blend door actuator(s) can move the blend-air door(s) in two directions. When the A/C-heater control pulls the voltage on one side of the motor connection high and the other connection low, the blend-air door will move in one direction. When the A/C-heater control reverses the polarity of the voltage to the motor, the blend-air door moves in the opposite direction.

When the A/C-heater control makes the voltage to both connections high or both connections low, the blend-air door(s) stops and will not move.

On ATC equipped vehicles, the A/C-heater control uses a pulse-count positioning system to monitor the operation and relative position of the blend door actuators and the blend-air doors. The A/C-heater control learns the blend-air doors stop positions during the calibration procedure and will store a diagnostic trouble code (DTC) for any problems it detects in the blend door actuator circuits.

On MTC equipped vehicles, the A/C-heater control uses a timer-type calibration procedure to check relative position of the blend door actuator and the blend-air door. The A/C-heater control learns the blend-air door stop positions during the calibration procedure and will store a diagnostic trouble code (DTC) for any problems it detects in the
blend door actuator circuits.

The blend door actuator(s) are diagnosed using a scan tool.

Now, since the blend air doors are providing AC to the front, that leads me to believe the problem is related to the mode door actuator mentioned prior. The easiest way to check it is with a scan tool, but it must be a scan tool which can read codes other than engine related and it must see real time data.

I hope this helps. There is a lot of information, but I wanted to provide everything.

If you have additional questions, let me know.

Take care,
Joe

Welcome to 2CarPros. Could you please list the vehicle information? I don't show a code 56 for an 05 Magnum.

Okay code 56 for an 07 = Recirculation Door Linkage Broken. Basically the system commands the recirculate door to move, the actuator counts how far it has to move to open/close the door. This code sets when that number is exceeded. This doesn't necessarily mean it is the linkage though it could be the actuator output gear is broken and it just keeps turning.

To test it and replace the actuator is easy. Disconnect the negative battery terminal, Remove the glove box from the dash by squeezing the two stops in to release them, then disconnect the stop cable and remove the box. Then behind it to the top right of the blower motor is the actuator. Three screws and the wiring connector remove it. Installing a new one you just wiggle it so the stub shaft splines into the inner post. It will calibrate itself as soon as you turn on the key.
While you have it out you can test the door linkage to see if it is disconnected or broken br reaching into the splined shaft the actuator connects to and gently rocking the door from one stop to the other. If it moves OK, replacing the actuator should repair it.
I hope it isn't broken as that repair requires removal of the entire HVAC housing and that is not a fun job.