"The horn started to come on by itself"
I have a similar problem when I am watching Countdown, or any other programme with Rachel Riley in it!
However, I would pull the horn assembly from the hub (if its an alloy one they are an interference fit with a rubber O ring and so should just pull out) make sure the wire going to the central terminal is not loose and shorting somewhere, after that it could be a trapped horn wire under the boss (that requires tools) or a short in the steering column electrical block connector. As a temp fix you could just pull the wires from the horn/s?
