PMA4x4
Adventurer
Also inside your engine compartment starting at the firewall on the drivers side is the vacuum ball. Start there and follow the vacuum lines around and I bet they are all dryrotted . I had to replace all of them on my 91 Burb before the system would work correctly. I couldn't figure out why the air was coming out of everywhere at once and never really got cold. After I replaced all the lines it actually worked properly and you could hear it switch between the settings. It also got freezing cold because now it had vacuum and would actually go into max setting.
I actually fixed the door issue and made my own replacement tab out of formed metal. I have already started the vacuum line replacement. A PITA task but needs to be done.