I did this on my GTX with great results, my headlights are bright and there is no dimming.

Use two relays and mount them as close to the headights as possible, cut your harness at the relay and run a heavy power source cable to the relays. I would upgrade the wires from the relay to the headlights for best results. I ran a #6 wire from the alt output to my relay box, I scavanged a relay box out of a junkyard intrepid to make mounting of the relays neat.

Madd electrical is a good source, do a google search for headlight relays as well.

These shoule help you wire the relays...