AC in the back blowing hot air

My AC in the front works good but when I turn on the fan in the back seats the outlets in the back blow hot air while the ones in the front blow cold air. I have all the knobs for temerature control set to cold. Any ideas anyone!!

Thanks
Saquib

ive replaced alot of the thermal expansion valves for this problem on the ventures, also could be temp door actuator at back hvac unit.