http://www.frontiernet.net/~jmray/F250ColdAC.htm

I did this to my 2001 F250 with Napa Heater valve #660-1867, 8 feet of vacuum hose, and a brass aquarium Tee. This will only shut the valve when in Max A/C mode. Once it is cold, I switch to A/C mode to allow the water to flow thru the heater core. It works great!