As I mentioned in a reply to Monoplex's post about customization, I recently installed a set of loud truck horn behind the legshield of my People 150. I got it taken apart right now for painting so I thought it would be a good time for photos. Ignore all the extra wires and bird nest, I've been playing with a lot of extra lights and do-dads.
I left the original horn also. I tied the new horn relay into the existing horn circuit and ran power for the new horns straight from the battery.
The black wire off the white plug is switched power, you can tap it for anything you want to turn on and off with the key.
Theres the other horn, a high/low pair. Their pretty loud and were only about $20 at autozone.