[purple gas]

Just a thought.. In my obsession with all things heater/ ventilation realated I was thinking that our trucks, unlike slightly newer vehicles don't seem to have any exit vents for the cab. Newer vehicles all seem to have someplace for air to escape so the ventilation system can "flow through". mid '70's trucks had a black plastic vent inside the door jam. I'm thinkin if a person put some type of vent with a one-way flapper type valve in an inconspicuos place it would let the air out in a controlled way. This would stop the air from having to sneak out any place it could , as through vent seals etc. What do you think? It would also improve heater air flow volume. Also, your problem may not be eveident when the truck is parked because the cowel vent area is a natural high preasure area. As speed increases air is forced into the cowel and gives a bit of "boost" to your heating system. If you've ever driven a truck with a blown heater fan, you'll know becasue if you drive fast enough you can feel air come through the defroster/heater. ( Not recomended)