My rear Air Conditioning unit started blowing hot air. Front AC unit works fine. on 2003 GMC Savana 1500
Make sure that the system is not low on freon, as this can cause weak cooling in the rear A/C unit.
More often than not, this is the rear AC switch not functioning. The two systems share refrigerant so if one is working fine and the other not, it wont be the refrigerant but rather the compressor not working for one reason or another.
i had this same problem..what happened is there is a small hose running from the rear ac where the heater control valve in the rear is located this hose in diameter is7/32 could be worn through which causes the heat in the rear to constantly run change the hose and check the door to make sure that is open located under the hood..blowing cold now 95 dollar repair