With any kind of air horn, this circuit will blow the factory 10A fuse as soon as you activate the horn button and the added switch is on. Well...maybe the 2nd time. How do I know this? I made the same mistake and I'm an Electrical Engineer! The air compressors for the air horns have a huge starting current, probably about 10-15A. I just followed the instruction that came with the air horn, which said the added relay was optional. Mine is wired with no added switch. Since I have only 1 horn, I used the unused 2nd horn connector to actual the coil of the added relay and then an 8A time delay fuse from terminal #30 to the battery.