More than once I have gone to adjust an air conditioning vent to find that the louvers have sunk into the frame. The culprit: the frame has cracked at the hole in the vent frame where the louvers pivot. (Click on the pictures to see a larger version.)
Oh! The pain!
Twice I've repaired broken vents in the following manner, and none of the repairs has failed.
Upper left: the tools. A cutting mat, some plastic cement (I used "Plasti-Zap"), a thin sheet of styrene from the hobby shop (0.015 in.), a metal straightedge, scissors, matte knife, pencil. Not pictured, a single hole punch.
Upper right: Make some reinforcements to buttress the hole after it is glued. Use the ruler to measure for the styrene insert. Cut a strip of the correct width, but longer than necessary. Set it in position, mark and trim with the matte knife and scissors for a clean fit.
Lower left: Mark for the holes. Use the hole punch to make holes for the pivot points on the vent.
Lower right: Repair the broken piece of the vent frame by gluing the broken parts together, making sure not to leave a build-up of cement on the outside surface of the frame. Next, glue the reinforcement piece in place on the outside surface of the vent frame (where it won't show when installed), making sure to use adequate cement around the hole. I added a second reinforcement to the other side, even though it wasn't broken, as a preventative measure.
After allowing adequate time for the cement to dry, snap the louvers into into the frame, and snap the frame into its hole in the dash.
Enjoy the satisfaction of another nearly impossible job well done.