How do I access the trap door on my '92 Firebird?

[Unrelelnting Optimist]

the heat will only come out onto the windshield I know there is a little trap door or a lever that moves when I change the position on the dial for heat selector. it must have come loose or broke but how do i access the trap door
NO THERE HAS TO BE ANOTHER WAY!!!!!!!!!!!!

 

[Zimgxp]

Make sure there is vacuum to the dampers. Look for broken or missing hoses.