If you don't hear the faint hiss of the vacuum pod / solenoids when you change selector positions, then you don't have a vacuum signal to move the blend door that directs the air to the different outlets in your cab. Often, the vacuum line from the engine compartment has been disconnected at the firewall (engine side) or at the vacuum source itself (engine). Check to see that it is connected. If your truck is similar to mine (85 Silverado), the port will be found on the firewall just left of dead center (as you look at it standing in front of it). Have you performed any work recently under the hood or under the dash that might have accidentally disconnected a hose?
1
