I’ve never messed with a Fly, but since no one else is responding: next places to check would be the wire connections at the handlebar switch, and then at the horn. If everything looks good in those spots, I’d look inside the headset where it meets the steering column to see if the horn wire has maybe been rubbing on something and is shorting out.
No idea how accessible those areas are on a Fly, but that’s where I’d look for issues.
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