Okay, so I'm planning on installing this twin trumpet airhorn into my car.
Off the relay it has a connection to +12 Vcc with a 30A fuse. Is this line direct to the battery?

I had air horns in my old ute but i forget how they were wired. I'm pretty sure your right when you say it should go to the battery, I found this diagram if it helps.
http://www.ibmwr.org/r-tech/oilheads/wiringbig.jpg

Exceptional. Much appreciated!