Stuck on defrost?

Check for vacuum leaks check vacuum source

Without any tests to interpret, the most likely suspect is a defective actuator. My cousin had two Rangers, and both had problems with them. We never took one apart to see if it could be repaired. He took both trucks to a repair shop to have those actuators replaced, so I can't tell you what is involved either or if it's something a competent do-it-yourselfer can handle. You might consider getting a copy of the manufacturer's service manual and reading through the replacement procedure to see if it's something you want to pursue.

That's for the electronic actuators. If you have the more common, and more reliable vacuum actuators, those will be spring-loaded to default to the defrost position when vacuum is lost, as in a broken or disconnected vacuum hose. That's for safety. Your feet might freeze, but your windshield will be clear. Look on the vacuum hose routing diagram on the sticker under the hood to see if they show a small vacuum hose going to a storage canister, then through the firewall.