for the electrical gurus..

15-20 amps usually does it, but put a 30 if it tickles ur fancy.lol

Seems like a good way to turn your harness into a fuse to me. Since the original poster doesn't state the amperage draw on his horn the fuse size can only be guessed at. Is it a replacement roadrunner horn or is it a jc whitney made in china knockoff? They will have different amperage draws. I would still run it off of properly fused relay regardless of method of trigger.