I have a 1989 F150 w/300-6 cyl. When it rain the windshield and the side windows fogs up. I change the heater coil but it still fogs up. I can turn on the defroster and start to ride down the street and it will clean up some. But soon as I stop it fogs up again.

Dofunny.

 

My windows would fog like crazy any time it was humid or even thinking of raining. It helps quite a bit to have the air contol switch set to "outside" air. In my case the vacuum operated switch which controls that function was stuck. I freed it up with some lubricant and got it working. A bunch of sand and grit had got in it somehow. With the engine running move the switch back and forth and see if the diaphram moves. Got my 5 year old son to help out there. Was a great game. May be your problem. Check it for vacuum too. It is located on the passenger side of the engine bay near the firewall. Basically on the air handling duct work.

Richard
88 f250 4x4 351 twin turbo

 

Basically when windows fog up, its best to turn the Air Cond on with the lever setting on defrost and the controls on cold, this way the windshield will become clear quickly, or wipe your inside windows/windshield with a defogger wipes.(your special brand...any will do).

 

The duct work for the heater and such is more than likely plug with debris, in my flareside i had a bunch of napkins, some tothpicksm and more, after i cleaned it out it was like a brand new heater.