Hmm, I am no expert when it comes to wiring, but fog lights generally draw enough current to require a relay. Hard wiring them directly into your low beams may overload the wires and cause a meltdown. If you are looking for a wire to activate the fog light relays every time your low beams are on, that's easy; just use a 12V test probe to identify any wire with "+" current when the lows are on.

I learned about overloading wiring when funny smells and smoke came out of my MGB's dash. . . .