Cyanide has the described the method that I would advise. Technically, this means that your fuel pump is turned on by 2 relays sequentially, but who cares. 12ga wire is good, and a 30A relay is, too. The "blue/red you cut at the connector side" that Cyanide refers to should be terminated/covered correctly. This more than likely is a 12V source (like most blue/reds in the car), so you don't want it shorting to ground.

My method would be to simply trace out the factory wires through the body harness and replace them with beefy 12ga. Hey, I've already modified the chassis harness once already....