I havent messed with that wiring before, but just off the top of my head, i know that the headlight fuse is under the hood and the fuse for the tail lights is in the cabin. I presume that the tail lights and fog lights are probably on the same circuit or at least activated from the same place, which means that the wiring most likely splits before going through the firewall.
That said... why bother? IMHO its not worth messing with the factory wiring behind the dashboard, its just too tight and easy to get messed up. its super easy to run wiring through the firewall, there is a very easily accessible grommet right in front of the steering wheel. No drilling required and protected by the hood and cowl overhang. You can even re-seal it with silicon if you are really concerned about water. I'm only 15 minutes from Collegeville, I would have no problem running the wiring for you someitme.
If it were me, I would put a waterproof relay under the hood that is fed power directly from the battery and going out directly to the new fog lights, and activated by the factory fog light switch. You can run a tiny 18 or 20 ga wire to the switch to activate the relay. Its easy to find a plug to match the factory fog lights, so you could even leave the factory wiring completely in tact, just cover it and secure it behind the bumper so if you ever want to go back, its simply unplug aftermarket, plug in factory.