If your fog switch is on the radio bezel. I would take it out and jump it with a wire. If that is not it. Check the relay. If none of those, you may have a wireing issue. Mabe a loose ground.
I seldom use mine, but had to just last night. I did notice that a couple of times when I hit the push button, the indicator on that button didn't light up. I'll have to look into that. One day when I have nothing to do... ;-)