Rear ac/heater not working. What could be the problem?

This depends on what stopped working - if the fan no longer blows, it's an electrical issue like a blown fuse, failed blower resistor or burnt up fan motor. If the fan blows but no air comes out of the vents, it could be a problem with the motor that opens and closes the vents. Can you provide a little more information? Ben from Quality Motors of Laguna Niguel