If your vehicle is showing rear a/c or heater is not working, here are the most likely causes and the parts you'll need to fix each one.
The blower motor resistor controls the fan speed for the rear climate control. When it fails, the rear fan will typically only work on the highest speed setting or stop blowing air entirely.
The electric motor responsible for pushing air through the rear vents can wear out, seize, or burn out over time, resulting in absolutely no airflow from the rear climate system.
This small electric motor directs air over the heater core or A/C evaporator in the rear unit. If it fails or its plastic gears strip, the rear system may be stuck blowing only hot air, only cold air, or clicking loudly.
SUVs and minivans have long aluminum refrigerant lines running under the vehicle to the rear A/C unit. These are highly prone to corrosion and road debris damage, causing refrigerant leaks that stop the A/C from blowing cold.
The physical switches or the electronic control module in the rear passenger area (or the master override switch in the front) can fail, often due to spilled liquids or electrical wear, disabling the rear system.
Sediment, rust, and degraded coolant can settle in the rear heater core, blocking hot engine coolant from circulating. This results in the rear system blowing cold air even when the heater is turned on.
The rear A/C unit has its own expansion valve to regulate refrigerant flow. If it becomes clogged with debris or fails mechanically, the rear A/C will blow warm air while the front A/C may still blow cold.