This is a quote from someone from some forum (can't remember).
"Okay..
The original horn wires from ONE side, you plug it into pin 85 and pin 86 of the relay. For my Honda the horns are seperate, one left one right but yours might be together, no difference. The other pair, just tape off and keep away.
Then, from the battery run a wire with a in-line fuse to pin 30 of the relay.
Lastly, connect 1 terminal of each horn to ground (maybe just a few CM of wire to the metal bracket it's mounted on, and from pin 87 of the relay, you wire it to both the horns... volia.
Relays should be RM7 odd. "
Hope this works for you :D