just like any other lights, wire the switch to a relay so it only has power when the lowbeams are on, then run a wire from the relay to the switch, and then another to the lights, done.
also now i have 2 factory fog light buttons, so one will be turned into a intercooler sprayer. ill have to make a custom cover for it from a rex to fit the button.
1999 Impreza Outback Sport w/ MY02 WRX complete swap