If you mount the Stebel on the rear swing arm in front of rear tyre then you are going to have to run some form of wiring from the front of the bike - I cut into my stock horn (which I left on) wires and ran them back along the frame under the tank when I did the pair valve removal, and placed the relay under the seat.
Hooked the horn wire extension to the relay, ran a fused wire from the battery terminal to the relay then from the relay to the horn. I ran the earth for the horn to the screw under the left side cover that holds the tool/manual holder to frame.
I glued a piece of PVC (6" x 1.5" x 0.5" thick) to the top of the horn and then mounted the horn to the cross bar on the swing arm in front of the rear tyre. The PVC gave me a flat surface to place against the frame and helped keep the horn in a verticle position. Then used really big cable ties to fix horn to swing arm - works well, is really loud and has not moved.
Have had it on the bike for about 6 weeks now.
There are other threads which will provide you with more info - just do a search on Stebel.
You can pick up a fused wire from Radio Shack, auto store or someplace similar - it is just a length of wire with a fuse mounted in line used to protect the relay and horn from any surge. From memory the fuse needs to be 20 amp I think. All I did was connect one end of this to the battery and crimp a spade terminal on the other end to connect to the relay - that's your power source in.