I installed a Napa Flamm horn on my 650, ($17 vs $40). I have also installed 35 watt PIAA lights on my SW-Motech crash bars, so.......... I cut the relay off of the PIAA lights and purchased two more automotive relays, (4 prong) from Radioshack. These two "new" relays I am going to use on the PIAA lights. I want to be able to turn the lights on and off independantly from each other. That frees up the original PIAA relay to be used with the horn.
The horn comes with a piece of flat stock that can be bolted to a stud on the backside of the horn. The opposite end has a 1/4:20 hole drilled in it. I removed the aft allen screw that holds on the rear part of the left fairing and shoved a piece of cardboard between the fairing and the frame to hold the fairing out. I used a propane torch to heat up the end of a rat tail file, (I bent the handle at 90 degrees), and when it was red hot, I melted a hole in the side of the plastic radiator shroud behind the radiator on the left side and bolted it with a 1/4:20 stainless steel bolt and Nylock nut.
The horn hangs down "inside" of the crash bars and right behind the left PIAA light. The switch is a Radioshack momentary button that is mounted inside the left hand guard at a location that I can "stab" with my left pointer finger.
Both the stock horn and Flamm horn work on my bike. The Flamm horn is connected directly to my Blue Seas fuse block mounted underneath my seat. I decided to wire the horn "hot". I can use it when the key is NOT in the ignition.
This setup should work for the Stebel horn as well.