Sounds like you have a short in the horn circuit. Unplug the horn and try it. If it still blows the fuse you have some tracing to do, perhaps the switch.
I’d bet its the switch. Might be wrong but I know on my 254 any damp or rainy day, forget about using any switches, almost always blows a fuse. One of these days I’ll either weather proof them or replace them. Seems like I never get to it…
Aubrey
God is Great, Beer is Good, and People are Crazy! '07 Jinma 254LE