Unless you are adding a very powerful second horn ( that reads one that draws a lot of current) you don't need a relay. I have wired up as many as 4 horns to the stock wiring and no relay. Two Hi pitch PT horns, and 1 high pitch and 1 low pitch Oldsmobile Alero horn all to the stock wiring without issue. When I added the 2 Hella german horns however I did use the relay in the kit. 6 horns would pull too much current for the stock PT horn circuit. I mounted the 2 Hellas behind the grille. now you can hear my horns and alarm for blocks. It actually hurts my ears to be standing directly in front of the cruiser to set the alarm. It chirps loud too.....
