does your horn work with your lights on. ive just been reading on another zaf forum about a recall about something called a cim which allows the horn to work without the lights on but as soon as you turn them on the horn doesnt work.
id say either the horn or the wiring to it then. im sure you can run a wire from the positive side of battery to one of the connections on the horn to see if the horn works. if it does it will be the wiring to it thats stopping it from working.