a/c vent selected but air blows through defroster

Hello, This is a mode door actuator problem. The mode doors inside the HVAC ducts are turned by actuators, which causes air to move through different paths to different vents. You will need to check your fuses first, then, listen for the mode door actuator to change the mode door position when changing vents. If the actuator is turning, the issue is either a coupling between the mode door and actuator, or the mode door has broken. These are all fairly common issues for many vehicles.