The six volt horn is very loud on twelve volts and I would guess that it draws even more current.
Connections: there are three terminals on the relay. With an ohm meter you will find continuity between two of them. One of these is the supply line and the other is the line for the horn button ground. When you apply power to the two terminals there will be a click and the third terminal will be connected to the supply line. You should be able to tell this using your volt meter.
