U have a engine vacuum loss to the AC mode control switch (this switches the different functuions, dash, floor etc.), pull the ac control and check the vacuum, should be around 18-20 inches with a vacuum guage, if it is replace the mode control vacuum switch, if not check the AC vacuum hose for a restriction or a cracked split hose inside engine compartment.

I had the same problem, it turn out to be the vacuum reservoir but to replace it you have to literally remove the A/C compressor and everything around it. But me being me I remove the right front fender and cut a hole big enough to get the reservoir out from underneath the compressor. I actually left the old reservoir in place just removed the vacuum line and insert it in the new one then mount it, case close it work well. thank you Ford engineers for making it so difficult to replace it.

You have a vacuume problem some where if access to a smoke machine is possible it's easier to find leaks.....

The hoses for vacumm from the engine are misplade or broken, check routing diagram and inspect hoses for cracks.