either your heater core is shot or the temperature switch in
dash needs replacing I had a 1986 Nissian Pickup that had the same
problem when I purchased it back in 1988 in Phoenix, AZ.
I figured this out driving thru the Rocky Mountians on my way to Ft. Knox, KY. The heater had been running for about an hour and I was still cold (good for staying awake). I looked under the hood and found the problem.
I hooked it up again and started to drive again and in a couple minutes, I could smell Antifreeze.
The old owner had by-passed the heater core because it had a leak. Someone may have done the same thing to your Ford. Hope this helps. Good Luck and Remember.
Change Your Mind, Not Your Oil.