heater problems

i have a 1990 Buick Le sabre custom with 83,000 miles. i have a issue with the heater not working. the fan works and i can hear it blowing. the problem is when i try to switch from defrost to other positions it doesn't switch. on high i can feel very little air coming from the front vents. but its not enough to heat the car. i was told to check for vacuum leaks but i was unable to see any lines disconnected or broken. can any one help thanks.

There should be a small (about 3/16") vacuum line from the intake to a fitting on the firewall or in a wire harness passing through the firewall. These are often damaged or disconnected. The control head needs to port this vacuum to the various actuators to control air flow. With no vacuum the default position is the defroster outlets.