Heat blows when the power is off.
 

My 2004 Pacifica will sometimes start blowing heat to the floor when the climate control is powered off! The ATC display is off but the heat is blowing to the floor. When this happens, sometimes if I turn the ATC power on and then off the heat will stop blowing, sometimes not and it will keep blowing heat without anyway to stop it. Most times the ATC works fine. The Chrysler dealer said it needed a new ATC control head $670. I bought one from the junkyard for $75, installed it myself , but I still get the same intermittent problem,so the problem can't be with the control head.
Appreciate any ideas to fix this strange problem.
Thanks
Rich