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Your car should have a set of vacuum solenoids (#2 in picture below) under the passenger side of the dash that control the modes (vent, floor, defrost). Start by checking for vacuum at the source hose (#3 in the picture below). If you have vacuum at this hose with the engine runniing, then check to see if the solenoids click as you change modes on the control panel. If the solenoids click, you will need to check vacuum output from the solenoids to the vacuum actuators that move the doors.
Picture from under passenger side of dash looking up. Arrow points toward front of car.
In the diagram below, #3 should be the connection at the engine for the a/c vacuum hose. This is near the throttle body.
This vacuum line will tee off and one side runs inside the car. The other side should go to the front of the car on the drivers side to the vacuum reservoir. This is mounted below the fender.
If the line that runs to the reservoir is broken then it could reduce the amount of vacuum available for the inside controls. Look down low on the passenger side of the firewall for the wiring harness. The vacuum line comes through the firewall with the wiring harness. This will give you a location where you can hopefully find the vacuum line and trace it until you find the break.