A relay is recommended. Run new power to the relay with a fuse in the positive wire (always run the negative wire back to the battery on our bikes with aluminum and steel frames). Trigger the relay off the power to the stock horn. Use a pair of FIAMM Freeway Blaster horns or a Stebel Nautilus air horn (I used a Stebel on the left side and a lo-tone Freeway Blaster on the right, wired together).
Feather duster tomorrow