Just did this mod on my 03' 6.0l. This will allow you to have high idle for your A/C when sitting or parked and for winter time warm up. Not sure if it works on all years of the 6.0, it did on mine though. Much cheaper than buying a AID and only takes about 15min to do, just thought I would pass along.
http://dan.prxy.org/Truck/Other/High_Idle/High_idle_mod.html
http://dan.prxy.org/Truck/Other/High_Idle/High_idle_mod.html