Either you have clogged or busted vent pipe or the actuator could be going bad.
Most likely you have a vacume leak under the dash.Check for lines off or broken.They will harden and crack with excessive heat,such as in the engine compartment.
If it blows only out the defroster vents : then check for Vaccume leak Rick ASE L-1 Master TransMichigan.com
The doors are moved by vacuum. You need ot trace the vacuum line that goes into the firewall on the pass side to make sure it is not broken.
could be a vacuum line off
My ac blows only in the defroster and not any of the other vents i have 98 ford windstar 3.8 v6
what would cause all power windows to only work once in awhile and defroster to go out on a 1997 dodge intrepid
The vacuum line that goes to the defroster is probably disconnected, possibly on the fire wall.
the heat blows out the defrost when it should blow out floor vents only
Loose belt - will sound like a screeching noise.
The flap is stuck open that controls when and where the heat goes
some thing is probley cloging the vents take it appart and ckeck.
its a type of connector thatstends to fall off in most cars you will probably notice the air is mostley comming out of the defroster that's usually located in the dash.