The rear A/C has a blend door that controls whether heat comes through or Cold. I suspect that that door is either warped or broken and stuck somewhere in the middle. To diagnose the concern, I recommend taking it somewhere that has access to a Factory scanner that can communicate with ALL onboard computers, with the scanner you can run the HVAC door calibraion procedure, what this does is run all the doors to their travel limits and sets the "Default" door stops. If any doors are warped or broken it will fail the test and a result will be displayed as to what door has failed. At that time the door will need to be replaced and run the calibration again until it passes.

If you want to just check it manually, you would need to access the rear HVAC unit and disassemble it in order to check and replace any broken doors.

There is one "Side Effect" that may have happened by adding refrigerant to 40 psi, it may affect the overall operation of the system if you added too much. So if the front a/c is not working well, you may need to have the a/c evacuated and recharged with the "Proper" amount of refrigerant by weight, that's the best way to make absolutely sure that there is the correct amount of R134 in the system....

Removal of Rear HVAC unit - Click here to view

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Robert