2002 Ford Expedition A/C heater

Most likely you have vacuum-operated mode door motors and 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.

Start by looking under the hood for the vacuum hose. It will be shown on the vacuum hose routing diagram. Somewhere in that hose will be a small check valve too, typically near the firewall.