Does yer AC vent droop down or fall into the outlet? Here's what I did to fix one on my 1996 T-100. I removed the vent module using a butter knife to gently work the vent module out. 
On the sides of each vent module are little tensioned pivot bumps that keep the vent from moving from their adjusted position. The tabs will break over time and give you the droopy vents syndrome. If the broken tab is still attached or intact, you can JB Weld a small backing to it and make it perform like new again.
Broken tab...

Scrap piece of metal

JB Weld to the rescue!

New support fixed into place

Like new again!

Make sure you allow the JB Weld to set up properly or you'll be doing this job again.
On the sides of each vent module are little tensioned pivot bumps that keep the vent from moving from their adjusted position. The tabs will break over time and give you the droopy vents syndrome. If the broken tab is still attached or intact, you can JB Weld a small backing to it and make it perform like new again.
Broken tab...

Scrap piece of metal

JB Weld to the rescue!

New support fixed into place

Like new again!

Make sure you allow the JB Weld to set up properly or you'll be doing this job again.