Heat blow-by

[Pender1]

I have an excessive amount of hot air blowing into my truck any time I'm moving. With the heater turned off and the fan off driving about 35mph, it's blowing out of the vent above the center hump with the same force as if I had the fan on medium. I live in SC, so this is getting to be a problem as it's blowing HOT air. Does anyone know where to start looking for what's wrong? I know absolutely nothing about my heater or the vent system. By the way, I don't have AC, just heat.

 

[Georgeb]

check to be sure the control cables are moving the levers/doors first.

 

[Pender1]

check to be sure the control cables are moving the levers/doors first.

It stops blowing when I stop moving, so it would make sense that a butterfly isn't closing somewhere or something like that. I'll get under the dash tonight and see what I see.