Relay? Are you needing to switch 72 volts on and off from the handle bar? Or do you need to convert the 72 volts to 12 volts that the horn and lights run on?
sorry, I've bought a 72v light, and a 72v horn to make things easy/compact...
but I don't think the switches on my handle bar will take much power, so wanted to power then on via a couple of relays, but I can't find any with 72volt coils in them