It has a rubber grommet that slides in from the side, often dried out and hard.
76 Camaro AC heater box6 by
Ben, on Flickr
I'm not sure what you've tried, and can't recall how mine was which is modified anyway, but there's adjustments in the length of the control rods, location of the pivot brackets, and the manual cable length from the dash. From memory one is closed under vacuum but fights the spring tension so will open again just sitting there.