from memory the horn wire is just a negative trigger, so when the wire is grounded, (eg to some part of the chassis ground) the horn should go. i have not the need for a horn relay, unless i put bigger horns in..? the horn relay you would want to put between the battery and the horn, if you were to use it.. also a horn you might need when the cars ignition is off, so the power should go direct to the battery via a fuse.