I don't know how common a problem this is, but on my '84 Eagle the two directional AC vents at the top center of the dash were very loose and flopped up and down. They would not hold their vertical position, so it was impossible to aim the AC stream where it would do the most good. Originally, there was probably a plastic on plastic bushing to add some friction so they would hold their set position.
I did NOT want to try to remove the vent assembly and fix it on the bench; all of my interior plastic is very brittle, and crumbles to pieces from a disapproving glance, much less actual handling.
So I went to Home Depot and bought 3 neoprene washers for 75 cents (about an 1 1/2 inch diameter with a 7/16 inch hole (Everbuilt #597 599). I cut the washers through on one side.
The vents themselves pivot on a spindle that is about 1/4 thick on the outboard side. I used an old clothespin half as a wedge to force the right-side vent as far to the left as I could. That gave me enough room to fidget one of the split neoprene washers over the spindle on the outboard side. I then repeated the process for the left vent, again slipping the washer over the outside spindle.
The rubber washers take up just enough space, and supply just enough friction, to restore normal operation to the vents so that they stay where you put them.
The biggest price for this fix was the time I spent picking through the 'neoprene washer' drawer at Home Depot, whose 60 or so compartments each contained a perfectly randomized sample of the 60 or so sizes in the drawer.
Hope this helps someone with the same annoying problem...
JB