Originally Posted by FiReDeViL
i doubt an f150 has a rear defrost
Actually the newer ones do have this as an option if you don't have a rear slider. Chevys do too.
My guess is this is caused from a vacuum leak. My parents have my brother'* 95 and it was having what sounds somewhat similer to your problem. Last summer their truck was having an issue with no power with the ac running. Also the ac would blowing hot air when you would accelerate and coulder air when you would stop. They took it to a shop and all it was was a cracked vacuum fitting. Try running the AC to see what happens.
I can ask but I'm not sure if they know what part it was.