The horn on our Ka went the same way, they can fill with water! There is no fuse. There should be 2 wires attached to the horn, they are tucked away though. Remove old horn, one bolt but tricky to get at. It can be done with the car on the ground, access under front bumper with two spanners. Make sure contacts are clean and check circuit with a made up test lamp. Test horn with fly leads from battery. If dead, replace. I bought one from an accessory shop for under a tenner. I had to make an earth lead with crimp terminals and wire to earth horn terminal to mounting bracket. When re-fitting point horn downwards as much as possible to stop water ingress. Hope this helps.
Danny Barrow, November 2006