Air Vent Problem

Hi all. I'm new to the forum and my name is Ken. I look forward to speaking with everyone!

For my '89 Firebird, ahat's the most common cause of air from both the heater and A/C coming out of the defroster vents only? It's probably a vacuum problem but I don't know where to begin to diagnosis and repair.

Thanks!
Ken

the cable that switches the vents under the dash is probly either broke of cought on something.