If this vehicle has a vacuum harness to the vent control system, follow the line through the firewall to the vacuum pump and make sure there are no breaks in the line. You will know from past experience if it was a vacuum setup by when you adjusted the direction you probably heard an 'air slurp' as you switched from floor to defrost etc.
The defrost door is operated by the Red & Yellow vacuum lines, Per 'Mitchell'. The Blue & Green operate the Mode door which closes the vents. Don't agree with blend door, unless you have a heating issue.