The stock wiring is good enough for a single 100 watt bulb, but not two. The stock bulbs are 55 watts each, 110 watt total. The current draw for 110 watts at 12 volts is 9.1 amps, and the wire size is 18 gauge. The 18 gauge wire is a little small, since the current rating is 5-8 amps for the 5-10 ft of wire in the circuit.
Two 100 watt bulbs draw 16.6 amps which is way too much for the stock wiring.
For any fog light change, I recommend that you do the following:
1.) Use a relay to eliminate overloading the stock wiring.
2.) Draw power for the lights from a fused battery connection.
3.) Use the stock wiring from the switch to power only the relay coil.
4.) Use 12 gauge wire. These changes alone will make the fog lights much brighter without changing the bulbs. The 12 gauge wire is good for 20 amps, and is what I would recommend for using two 100 watt fog light bulbs.