[GoreMaker]

Ah hah, just got confirmation from the service manual. When AC is on, there should be no flow from any vents except the AC vents. So that door needs to be adjusted so it's sealed with vacuum on the white tube, as I suspected.

[Trevor78]

Good pics. See first and 2nd for spring on door flap that keeps it shut, vacuum pulls it open. The staples normally hold a sponge foam seal onto the door, always long gone.

3rd pics you can see the spring at top right on the pivot post.

[Trevor78]

Probably all engine types had their version of the retrictor fitting in Firebirds since there was no heater tap valve. I was only just posting info and pics of the Olds version on another forum.