I just did this yesterday too. I used a FIAAM , $15.54. I extended the wires by making short jumpers so I could face the horn down so water wouldn't fill it up. My instructions said polarity didn't matter, so I just hooked the 2 wires up and it worked fine.I didn't use a relay as I only used the "low" horn and I think the switch will handle it , 6 amp max draw. . I hardly ever use it but I wanted something louder for emergencies.