This vent is under the dash in front of the passenger seat. It has a hinge on it but doesn't have a way to open unless it is pushed on. Unless the fresh air comes up & somehow opens it. Anyone know how it works?

That looks a lot like the vent in my 86. On my coach, the heating/cooling system is (supposed to be) controlled by vacuum.

When I got the Barth, the dash air didn't work and the vents had vacuum leaks all over. I never cared to fix the AC so I still haven't done much with it. I disconnected the main vacuum source to stop the leak. This left all vents open all the time on my coach.

Meanwhile, the vent you describe blows fresh air whether I want it to or not. I still haven't diagnosed or studied the system very well. I use a couple rags to block the front airbox opening when it is cold outside. If I apply a constant vacuum to the vent hose, the vent will close, with no vacuum it is constantly open.

I plan to use a vacuum source to identify which vacuum hoses open which vents and eventually get the panel controls working again.

Unfortunately, I have found very little info on the GM vacuum driven climate controls. I will share what I can find.

Meanwhile, if you have the vacuum driven system, check the vacuum hoses going into the heater box area. They may be causing problems if they leak. Nearly every hose I checked had cracks and/or leaks.