Just cut the 3-prong connectors off the headlight harness and splice two of the wires into the output terminals from the relays. The third wire is a ground. A couple of inches of small wire spliced to the 12 gauge won't make much difference. A plain old insulated flat blade female terminal will slide right onto the headlight male blade terminal if you don't want to mess with doing any splicing. In the past two weeks I've wired both my motorcycles for high output halogen bulb headlights, using Bosch relays and 12 gauge wire. They really light up the night! I've also got 55 watt LED side lights on both bikes.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!