Iceman,
I assume you got the message already, but add the simple relay. I had a '74 Yamaha TX650A to which I added a horn similar to a Stebels(sp?) Nautilus. I omitted the relay and soon had to have the wiring harness replaced after one long blast. I had just given my Triumph/Norton buddies a hard time about their Lucas electrics when we took off from a restaurant. I blew my horn excessively at an idiot, then about 10 seconds later, they were blowing their horns. I looked back and saw my smoke escaping from the tail light assembly. When I looked ahead, smoke was also getting out on the handle bar controls and headlight. The bike abruptly dies and they got a laugh. My main fuse was intact and I must attribute the wiring burnup to the lack of a relay. Our more electrically adept members may be able to explain. But everyone knows that electrical stuff don't work if the smoke gets out.
Get your horn and add a relay.
