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Here's a link that might help, from when I asked a similar question. http://wildguzzi.com/forum/index.php?topic=92438.0 I'm sure you'll have many options available as your bike has good places to stash one.
Here Canadian Tire sell a pair of Fiaam Freeway Blaster snail-shell horns for about $35 one Hi, one Lo complete with a relayThese are not Air horns, just the regular electromagnetic variety, made in Italy so they are Guzzi approved, they also have a factory in USAThese are really loud but ONLY if you give them a good supply and use the relay they come with.I stuck a pair on my EV which has a factory supplied Horn Relay but it's not wired with heavy enough wire to drive them to full potential, they weren't very loud.Battery to in-line fuse to Fiamm relay to Horn is the only way to go.Use the original horn wires to power up the relay.http://www.webbikeworld.com/motorcycle-horns/fiamm-freeway-blaster/I have put them on all my Guzzis, just put a pair on my Griso, they just fit under the tank where the originals were, I had to make special brackets though.The California II came with Fiaams as stock, that's where I learnt how good they are.BTW, be sure to mount so they are self draining, a few drops of water will silence them.
I like these horn threads. I�m curious what you guys use your horns for. I have never used mine despite commuting 60 miles per day in Chicago traffic. We definitely don�t have friendly and courteous drivers. Wisconsonites call us FIDs - F�n Illinois Drivers. Is the horn to alert drivers to your presence, or are you waking up texting drivers at green lights? I try to drive defensively and remove myself from any situation that seems dangerous asap. I wouldn�t want to rely on a horn blast to get an inattentive driver to react positively. I do agree it�s dangerous out there- the number of drivers staring at a phone or otherwise distracted in traffic is frightening. I fear a horn blast can be seen as an act of aggression, which is the last thing I want when on a motorcycle surrounded by 5000 pound cages. One twist of the steering wheel and you�re toast. Who knows what state of mind that driver is in. That text he is reading might be from his wife telling him she is leaving him for her yoga instructor.