It's been a while since I did mine, but I think the horn wire was either green or yellow...or possibly both. It is quite easy to find, just pull the passenger side battery tray and you will find it down below some sheilding.
The first horns I had were the tankless kind...they sucked. I had them hooked up with the stock horns, made it even worse. Even when they were alone, they just didn't sound good.
I got them off there pretty quick and got some Hadley twins mounted under the cab on the pasenger side. They are nice and loud...very nice. My air storage tank is under the bed on the passenger side. The compressor...I'm not quite as happy with it. It is currently not working, I don't know if I can try to fix this one again or just bite the bullet and buy a new one.