Ian, Did you get hit with the bad weather?
If the instructions that Bob linked to don't work, use and ohm meter. Two wires will be open and those hook from ground to the side of the horn not coming from the battery. The other two wires will have some small resistance and those will be the activating coil. Hook one to the battery and one to the horn switch. In both cases polarity does not matter.
Another way to hook it up is for the two wires that are open, one to the battery and the other to ether side of the horn terminals. The other side of the horn terminal will then go to ground. That way the horn does not have a constant hot wire.
