I installed a twin tone, 139 decibel air horn under the hood, right near the front grill, facing outward.  Used in conjunction with the stock horn, this thing is really LOUD!  The horn was wired through a relay, installed under the hood, and still runs off of 12v power.  I put a red illuminated switch, just to the left of the shifter, and I wired it to stay illuminated while the car is running.

The horn is Stebel Nautilus Compact Mini Air Horn.

Wiring Instructions: Pinout: