[Maurice Johnson]

On many cars nowadays (especially non-US built), the horn and headlights are hot-wired and protected by a fuse. The horn button or light switch makes the ground to complete the circuit. I see there is no reference to a horn relay in the Haywire description of your harness. If you have tied into a ground some where, the horn will blow as long as the battery is connected. The same thing will happen if a relay (used for the horn) has welded up the contacts, and is permanently "making". Seen it happen many times.