The bumper looks great!
For the factory for light switch, I think the best way to do it would be to run all new wiring and run a relay like normal, but switch the ground of the relay instead. I think the factory fog switch is made to switch ground only.
This is how I had mine wired up. I dont remember what wires are switched on the factory switch so you will have to use a meter to find that out. Maby someone here will know.
Hope that helped....