Heater vents

I own a 2000 chevy astro van. and the adjustment to direct the heat/ac to the diferrent vent is stuck on the windsheild vent. my question is what changes the vent location

A blend door.

The control linkage my be disconnected or the door may be jammed.

where can I find the door and or linkage.

If the air is only coming out of the defroster vent and it won’t change then the system is in default mode. This means you lost vacuum to the heater control, vacuum controls the actuators, if the vacuum source is lost the system defaults and just blows out of the defroster vents.

Have you had any work done under the hood lately? Open the hood and listen for a hissing noise while the engine is running.

The blend door is in the duct and to get to it requires partial (or a lot) removal of the dash to get at it.

Check out what willey said before tearing into the dash.