I have a F150 ford 2003 and last week the AC started blowing out the defrost all the time. It started switching back and forth when I would let off the gas it would blow out the ac vents then go back to the defrost now not even the heat will bow out any where but the defroster.

 

Check the little vacuum hardlines behind the battery.

 

My wife's explorer did that and it was a vacuum leak.

 

My wife's explorer did that and it was a vacuum leak.