My rear AC blows hot air when front AC is turned on. The front AC blows cold air.
check the charge.
All vehicles know of with rear air except giant RVs use the same compressor and condenser for both the front and rear units. Therefore, those items are okay, and the refrigerant lines and rear evaporator are also intact. If not, the front a/c wouldn’t blow cold air.
Check the mechanism that selects between hot and cold air. Telling us what kind of vehicle it is would help us give you more detailed info.
Rear AC’s typically use expansion valves for metering. I will make my WAG and say expansion valve.I know we dont have the model.