that is actually how it works, as far as i know (fogs use juice from the headlight circuit). there are many write ups on it (Corral, etc). i would highly recommend using a relay on the fogs.
i dont have faith in the stock wiring, so i did relays on lows, highs, and fogs.
if you just splice into the headlight switch, make sure you dont tap into the ISO circuit. it will fry if you use it to power the fogs. as i recall, the fog fuse (15???) is hot with the key on only or hot with the headlights on only, so you could tap into that fuse and be good.
very easy to rewire the stuff.
good luck.