Just a few things here, you probably know all this already, just trying to cover all the bases:
1) unless you "modded" the relay prongs, your fogs will only work with low beams on.
2) This will sound dumb, but try both switch buttons. You never know.
3) Get a volt meter or test light, and check to see power at your switch. The hot lead should have power when your low beams are on.
4) If you're good there, turn the key on, but don't start the car. Turn the low beams on, then push the foglight button. You should hear the relay click on.
5) If you hear the relay click over, use your meter/light, and check for power at the light plug. I'm betting you will not get this far, since the problem is with both lights.
hth
Brian