Asking for a friend

He has a 1999 Mustang where the air is only coming out of the defrost vents. He hears clicking under the dash when selecting between vents but the air only comes out at the defrost vents.

 

Bad blend door actuator. It's a pretty cheap (<$50) part and an easy fix...if you can reach the part. They're buried in the dashboard. Not sure exactly where in a '99 but in my 2011, they're behind the glovebox and a really tight fit to reach with a wrench. That '99 probably has two and if you get close to where they are, it's easy to touch/hear which is bad. Good luck!