I have a '01 GMC Yukon. The rear air conditioning works, but when it comes to(rear)heat…I’m out in the cold. Any ideas, if so, where in the vehicle are your ideas located? 
The rear HVAC system has a mode actuator controlled from the main HVAC controls on the dash. This actuator switches between the heat or AC mode. There may be a problem with this actuator.
Tester