Based on that update, I'd say you may have some sort of "blend door actuator control malfunction." Is your temp control electronic, or vacuum solenoid driven? In my XD2 if I key on all the way but don't start the car, I can hear the electronic stepper motor or whatever make a little "buzz" with every detent I turn the temp gauge. If you do the same in the GTI, do you hear a little buzz, or a small whoosh (vacuum noise)?
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