T800 hvac controller, defrost inop

[joed2323]

2009 t800 daycab, hvac controller on the dash will not switch from front/floor vents to defroster.

I actually swapped the hvac controller out of another 2009 truck since they cost $777 and it worked, had full function of the defrost/vent .. Two weeks went by and now it's stuck and will not switch just like the first time I swapped it.

So tried swapping with a 2011, and it still is inop.

So im guessing their has to be something else the electronic controlled hvac controller communicates with to switch from vents to defroster. Does anyone know if their is a resistor or relay in the dash somewhere?

Looking at the fuse panel their is only a fuse hvac con, and hvac motor fuse, obviously these are good since the controller has power but will not switch to defrost, what am I missing or overlooking???