Vent problems

Your HVAC system is actuated by engine vacuum. If the system will only work through the defroster, then you have a vacuum leak. Check for leaks both under the dash as well as the supply hose from the engine. Somewhere you have a leak and it is not allowing you to change the air flow direction. If you start the engine when you are looking, you may get lucky and hear a hissing noise from the leak's location.

Let me know what you find.

Joe

Check for a vacuum line that would go through the firewall to under the dash. Sometimes they break in the engine bay, and dont allow vacuum to make it to your control head.