2000 S-10 Heater problem

I have a 2000 S-10 EXTREME. It has all the toys available. When the heater control is in the foot/lower position and the temperature is cranked all the way off (to the left) heat continues to enter the cab. Chevrolet completely removed the heater/AC unit, dash ect and reinstalled with the same results. How do I stop the heat from entering?