I haven't had a BX in my hands so with that it would be either the battery or quite often the fuse block will have a positive post. The switched power can be pulled from about anything. I'd take an accessory type switch instead of something that is only hot when running. You want to be able to shut the machine off, turn the key to the on or accessory position and cycle the valve to relieve any pressure. Poke around at the fuse block or ignition switch with the test light to find something. Since the relay exciter leg pulls way under an amp it shouldn't cause an issue.