The internal-to-the-dash vent doors were giving me a fit recently. I hadn't used the A/C for 3-4 years (top down) and the air wasn't getting cold this year. One day, it wasn't working at all. I ran it on High Heat and cycled the Feet/Face/Defrost/Combo switch a number of times in frustration.

It works like a charm, now. One or more of the doors weren't moving correctly.

And my windscreen doesn't fog up any longer...