No defrost?

I have a 99 Jeep Wrangler Sport w/ 162,000 miles. The vent system consists of 3 dials, one for the blower fan, one for the temperature and one to select which vents, A/C and defroster (the defrost setting automatically turns on the A/C pump too).

Over the winter I had to replace the fan speed dial because it kept shorting out. This week, I discovered that when I turn the selector dial to either of the two defrost settings, the fan shuts off completely. But it works at all speeds with the selector in any other position. what is going on?