Yeah, what GDR up there said. Take a fused 20A feed from the battery via the relay and use the connectors from the horn relay in t'fuse box as the leads to the horn. There's space in the fuse box for the horn relay.
I can't recall which connector goes where - I worked it out with a multimeter to see what was earthed and what was "earthed-when-switched-on".
Alternatively, and it's what I'll do next time, put it in the "For Sale and Wanted" and get a Magnum 80, which is a straight swap for the standard horn...