OK I think I'm going to hold off on this for a bit until I have a better understanding of what I need to do. This relay I have is rated for 30 amps and the horn specs says it draws up to 18 amps. I think the optoisolator sounds like the best idea so far to avoid frying the Arduino. I just don't know where I can find one that would handle 18 amps. All the ones I'm seeing are rated much much lower than that.