Defrost vents will not adjust temperature

Just to be sure we are on the same page; the temperature works correctly at all times except when you are using the defrost? At that point it goes to full hot or cold? What do you have to do in order to get it to switch to the other temperature? Is there a certain point at which it flips temperature?

Also, it appears you have a manual temperature system. Meaning you have levers, and it moves cables?

If so, we may have an issue with one of the cables or levers in the control assembly. I am attaching the process below on how to remove and inspect it.

So, let's start with this and go from there.

Thanks