Done right, the headlight relay harness does not require cutting the factory harness. The tough part is finding the one connector that allows for one factory headlight connector to plug into the back of a connector that also plugs into one headlight, usually the one closest to the alt.
madelectrical.com has good relay harness info, but I just noticed they've exceeded their bandwidth. Prolly because another headlight thread with mad linked came to the top today.