Ever since it got cold here in Wisconsin I've noticed that my defroster has become a refogger. It seems like there's water in the heating system that causes the windows to fog up every time I turn on the heat. I don't think it's the heater core. I've experienced that before and I remember the strong smell of anti-freeze and the slimy vapor it deposited on the windshield. I think this is rainwater. I'm hoping there's a drain in the firewall/bulkhead that's just plugged up with leaves and not a structural issue (read rust). Any ideas on checking it and cleaning it? -- Matt

 

It's partially caused by hot air can contain more moisture. When the warm air hits the cold windshield, the moisture condenses, much like steam condenses on the bathroom window during a shower.
Try lowering the windows 1/4" or so, so that moist air from inside (including from your own exhalation) can vent out of the truck.
Try switching from fresh air to recirculating on the controls (or vice-versa).
Try running the a/c if you have that- the cold a/c evaporator will condense moisture out of the air before it goes up to the window, but you may have to fight for a comfortable temp in the cab.

I've found that on my 91, the combination of rolling a window down slightly, putting the vent on recirculating and fan on high (it doesn't have a/c) keeps the fog to a minimum.

 

I just blast outside air and full of defroster/fogger/whatever..
It doesnt fog up at all.

 

I would still place my bet on heater core.... i just replaced one wednesday. When it started a few weeks ago there was NO hint of antifreze for the first week. It was just foggy.

 

Oh and like abecedarian said, i need to crack my windows a half inch or so but i like to run the vents on fresh.