Your mention Factory ignition? The stock distributor and box need to be out of the equation. So just clarifying that. Pending year of vehicle you may have a 12v open slot on the fuse box. I havent heard of interference issues with the blue/brown method, but just my taste, I would much rather find a simple 12v source off a fuse spade, or even a switch under the dash. My 1974 had a random female butt connector coming off the fuse box. Perfect switched 12v.