Tips on replacing the stock horn on a BV500 and going from beep-beep to a loud honk-honk. I used a single horn from the package listed below. It fits nicely and one horn only draws 5 amps; the fuse is rated for 7.5 amps.
CandlePower Electric Horn Kit
Because the new horn is larger, it needs to be relocated slightly to the side of the original horn. This requires about 6-inches of extra wire for each of the two wires. It's a good idea to pick up the electical wire before starting. It is the same thickness as the wiring for the taillight and rear blinkers so you can simple eyeball that wiring. Buy six inches of black for the ground and six inches of a color for the other wire.
When you get to the point where you are connecting the wires; you can either splice them together and use electrical tape or use a connector. If you want to use a connector, you should buy two before beginning the installation process.
The horn is located under the front panel (honk the horn). There are a number of screws holding the front panel on and they are rather obvious with two exceptions:
 You need to pull up a corner of the rubber floor mats on both sides to expose the screws. The part to pull up is the inward section near the fuel tank toward the front where you put your toes. A little rubber flap tucks under and holds it in place. Pull up that corner and remove the screw (on both sides).
 On the front panel, gently pry off the chrome Piaggio logo. It snaps into place. There is a screw under that cap that needs to be removed.
Everything else is straight-forward. Remove the panel, unbolt the stock horn, re-locate and bolt on the new larger horn, splice the wires together and attach to the new horn, test the horn, replace the front panel.