I'll share my headache (and solution) from my experience with the vents switching to Defrost under open throttle. I started at the intake and worked my way to the Vent control checking/replacing all the vacuum components (lines, valves, fittings, etc.)
Well, it's always the last thing you check right??
Cruise control. There's a vacuum line that runs around the front of the intake and ends up under the battery at the cruise unit. That line had disintegrated due to some battery acid getting on it. Swapped that and the vents work perfectly now.