Why is unit blowing warm air out of rear vents

Hello, the rear of the vehicle has it's own controls and own temperature blending mechanism. Since you're getting cold up front, it's reasonable to expect it's not a refrigerant problem you are experiencing, but more a control issue. Focus attention on the rear climate control system and see if the blend door motor is working properly. www.repairpal.com/repair-shops Check that link to find some shops near you for assistance.