so are the 'mounting brackets' supposed to move? Are you sure? I took my vent off (it's '50 1/2 ton) to put the gasket on, and am trying to get the vent back into place but am having a rough time of it (it won't close all the way when I pull back on the under dash handle). But it sure looks like the holes that the vent bolts through connects to 'brackets' that don't move. I'll check again; anything's possible.
