The inside of the windshield of my 2004 Ford F-150 ices over in the winter time along with the outside of the windshield. Apparently Moisture is entering the cab from somewhere. Door seals all seem to be in good shape. I had the same problem about 5-6 years ago and took it to a ford dealer. They said that water was leaking through the light on the back of the cab above the bed. Maybe called the cargo light. They said that water was running from the light through a wire trough or conduit going under the carpet down the right side of the cab under the carpet. The water would freeze and thaw releasing moist inside the cab although the carpet did not feel wet. That corrected the problem until it reappeared about10 days ago when the weather dropped down into the high 20,s. I resealed the cargo light with silicone but I still have the same problem. Any suggestions that you may offer would be greatly appreciated.
The rear cargo/high center mount brake leaking is a common problem.
Now that you have re-sealed it, you need to dry the carpet and padding.
Are you sure the HVAC is set for Fresh air, not Recirculate? And does the AC compressor come on when the HVAC is set in Defrost mode?