Annoyingly, I can't find the tread I saw a picture of the dismantled AC controls - think it was on my350z, thread about swapping all the LEDs. But, I do remember that there's sort of two rings of copper contacts on the board inside. The knob presumably shorts across different sets so control the speed.
Maybe you've got a bit of dirt in there stopping it working? Though if it is that, I'm not sure how easy it'll be to clean...