Chevy Celebrity- heat blows through dashboard vents only

Hello- I have a 1990 Chevy Celebrity wagon with a vent problem. The air will only blow out the dashboard vents, so I’m getting no heat down below, or air to the defroster. Does anybody know how to fix this?

You’d have to check to be sure but the vent controls are probably done by vacuum lines. On the back of the switch is probably a bundle of vacuum lines, and the vacuum supply will originate someplace under the hood & pass through the firewall. You may have a supply line problem or a problem with the feeds off the back the back of the switch. I’d start by just checking around the firewall under the hood for a broken/disconnected line.

Thanks cigroller- I saw some small diameter clear plastic lines under the dash last year, but none of these seemed to be disconnected. I wonder if they operate the vent controls. I wouldn’t know how to access anything immediately behind the switch panel.