1995 Other GMC Models Rear Air Conditioner is blowing hot no

I have a 1995 gmc 2500 vandura, customized by Explorer. It has a rear air conditioner with the blower control up front and the temp. Control is combined with the front (1 control for both). The front blows cold but the rear blows warm untill the engine gets hot, then it blows hot. The rear fan is mounted behind the heater core which is behind the radiator hoses. As those hoses get hotter the air from the vents gets hotter. I cant figure out if something is not engaging or if I need to add insullation to those hoses, or what. Please Help Me.