Any of the following..................
1. A bad a/c heater programmer.
2. A vacuum leak.
3. A bad a/c heater controller.
4. A bad interior door actuator
the heat blows out the defrost when it should blow out floor vents only
on your car it is a defalt mode for air to blow out defrost when there is no vacumn from the engine so you should look for a broken vacum line or see if it has come unplugged on the engine.
The moveable "baffle" inside the dashboard is not redirecting the air flow as it should. It's operated either by vacuum or an electric motor, in either case, repair is going to require the removal of the dashboard.
2003 crown vic air conditioning vent dont blow
The AC does not blow on the main front vents. Only rear and defrost.
Because the air selector died.
you have a vacuum leak. defrost is the default when a vacuum leak is present.
Perhaps a stuck door in the HVAC system or a blockage in the ducts.
You probably have a jammed or otherwise defective vent door.
depending on what year and make it is, the most likely issue is that you have a valve that moves to divert air to the upper vents, floor vents, and defrost vents and it is either stuck in the defrost position, is broken, or a cable/connector has come loose. The valve is usually in the center of your dashboard up against the firewall. The air flow direction in many vehicles is operated by vacuum controlled servos. If the vacuum supply is cut off, the system reverts to it's default position which is the defrost mode. My guess would be that the HVAC vacuum supply line under the hood is disconnected, pinched or collapsed.
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The vacuum supply line from the engine is disconnected or broken. When the vacuum supply is cut off the system defaults to the defrost mode.