TL;DR - Everything is working again.
I replaced one of my Fiaam Freeway Blasters - that wasn't working at all - with the original horn, which I'd saved for some reason when I replaced it. Everything seems to be working again - albeit not quite so loudly - and no more blown fuses. I'm pretty sure that non-working horn was shorting out and blowing the fuse.
In this Series
My Horn/Hazard Fuse has Blown Twice (Resolved)Apr 14, 2021