If you go to the link I've provided below, it shows how to use relays to drive lights on a classic mustang. It works the same on any Nissan (or any automotive application) also except you only really need one relay instead of two. Instead of using the head light switch to power the relay coil you install a switch on your dash that's powered by the ign source you said you already have determined and wire that in as the source of power for the relay coil. Using a relay is a much safer way to add a high current device.
Brighter Headlights