To make this simple lets do this:
Hot wire from horn goes to 86 (trigger)
Horn ground goes to ground to new Horn
Ground from relay goes to any good ground 85
In line fuse goes to battery. You can get a inline fuse at radio shack if ya prefer with or without wire installed. this go's to 30/51
14 awg wire will do just fine for the horn, you can get cut to length at auto parts stores, lowes hardware, opr a small spool at radio shack
I always use a 30 amp relay. There are 2 trigger inputs on them. I will use both through time. On my bike I have my radio to 1 and my cig lighter on another. So If you only use 1 for the horn you will have another for something else. However if you do add 2 inputs you need to add separate fusees for each.
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