here is a link to Chet Walters' page with basic relay information on wiring up a relay. The OEM horn has a wire that has 12V applied when you press the horn button and a ground. The wire that has 12V applied will be your relay trigger. I mounted some el cheapo air horns underneath the swingarm and pulled the trigger wire back from the OEM location and spliced on a longer wire to get to the relay which I mounted underneath the right side cover. You will need a fuse inline to the relay, a ground for the relay which you can pick up underneath a bolt on the frame, a trigger lead and an output from the relay to the hot lead of the horn. You can test it all out before your mount everything permanent.
I took the front and rear tank bolts out and moved the tank over to the left and ran the wire up underneath the right side. If I were to do it again I would just remove the tank, it is not that hard.
PM me if I can help.
http://www.rattlebars.com/mtz/basicrelay.html