Defrost not working properly

This is most likely an issue with the defrost actuator. On this truck you have an actuator for just defrost so the control module can try and regulate the moisture on the windshield to prevent it from fogging.

Here is a guide that talks about this issue. It is not exactly the same but covers similar operation.

https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position

However, below is info from the manual on how to replace the actuator which is what I suspect the issue is. What I would do is remove the actuator and move the door manually and make sure you have the proper amount of air.

If you do then I would replace it and retest.

Please let me know what questions you have on this.

Thanks