2004 Chevrolet Silverado climate controll vents

Most likely your mode doors are operated by vacuum motors, and the vacuum hose is broken or disconnected. The system has spring-loaded doors to default to the defrost position for safety. Your feet might freeze but the windshield will be clear. There's a vacuum hose routing diagram on the emissions sticker under the hood. Look on that for the small, hard plastic hose going to the firewall and be sure that is not broken.

The mode door actuator in this truck is electric, not vacuum.

You can't really manually move anything while the motor is on there and you can't remove the motor and do it because the linkage operates about 3 door simultaneously. The one thing you may be able to do is help the motor along with your hands while moving the mode controls and then leave it in the position you want. The actuator is on the end of the HVAC housing on the driver's side. You should be able to access it from under the driver's dash.