A popular upgrade is to replace the mediocre factory horn with a Stebel air horn. I was getting ready for a our 2007 bike trip and I decided I might need some more horn volume.
I got the horn off Ebay for around $40. It includes a relay and poor instructions. There is better info on the Gadget page.
The horn can be installed as one unit or separated. I decided to separate it.
To separate the horn there is a clip (red arrow) and a fitting (green arrow) that must be displaced, then the horn and compressor slide apart. You must purchase a fitting (purple arrow). I got this one and the 5/16" tubing at Lowes.
I removed the factory horn and attached the factory wires to the supplied relay's trigger terminals.
Here's the wiring diagram for the Stebel horn. The air compressor draws ~14 amps so a relay is necessary. I ran a fused power wire to the battery.
I clamped the compressor to the frame and modified the stock horn bracket to accomodate the horn relay and my auxiliary light switch.
I moved the displaced factory choke (that I've never used) to the other side of the bike.
I zip-tied the horn under the fairing with it pointed forward. Eventually I plan to make nicer mounts for the horn and compressor.
The Stebel horn is MUCH louder. You can lightly tap the horn button for partial volume or lay on the horn for a full blast.