AC and Heat Flow Only At Defroster/Windshield

This is a common problem with vacuum-operated heater controls when the vacuum supply hose is broken or disconnected. Most systems are spring-loaded to default to the defrost position for safety. Your feet might freeze, but your windshield will be clear. Look for that hose under the hood and inspect it. There will also be a small check valve inline with the hose. They do not cause much trouble so that should be a last suspect.