I believe it is a low vacuum promble, it only happens on the highway when it is working hard, around 2000 rpm, you can hear it switching on you, when you let the peddle off it will go back to the floor heat, but when driving normal it will always go back to the window defrost, why?
I'm not sure if this applies to your vehicle but often a vaccuum reserve tank is used in the system to eliminate this problem. If you have one, make sure it's not leaking. If you don't and were supposed to, someone may have removed it. There also may be a check valve in the line somewhere that is malfunctioning. This used to happen on older vehicles with vaccuum wipers. They would almost stop when going uphill. It's the same thing, just on a different system! Could also be a slight bleed-off from a plenum door motor that is not bad enough to not work, but just enough to react to low vaccuum caused naturally when engine is working hard.
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