So, purchased an air horn that requires the compressor, yada yada; irrelevant. The switch were using is http://www.dormanproducts.com/p-8696-85962.aspx
it lights up when "on" and has three terminals (Ground on the far right, A/C On in the middle, and Supply Volt on the far left.
Well, I wanted to run that horn and the factory horn to the switch so I could use both. We have terminal 86 wired to the hot wire from the steering wheel button, terminal 87 is wired to the factory horn. The 85 terminal is grounded, terminal 30 is wired with a fusable link to the battery to get 12v power. What the idea was? Wire both the compressor and the factory horn to terminal 87 and have the switch be spliced into the wire between the compressor and terminal 87 (so with the switch on, both horns will sound, with the switch off, the power to the air horn will be cut so only the factory will sound.) What we did was everything above, like connected the terminals and what not, but we didn't connect the compressor (we just wanted to see if the switch was in the on or off position, and see if it had any effect on the factory horn.) The switch lights up blue when "on", and when I first hit the horn, it lit up and the factory horn sounded. I switched it to off, hit the horn, ..dead short. Does anyone know why this might've happened? Did we forget to ground something, put wires into the wrong terminals, not use a strong enough fuse? (30 amp) We're mind boggled.