I have heard (and seen) a few people that have does this as pranks and always wanted to do it to someone myself! How would you go about wiring a car's horn to the brakes so that everytime you mash them the horn goes off?
I did the same thing to someones turn signal once. Not to hard at all. You just have to make sure the polarity is correct. I think a mustang horn rests at 12V, and is activated by ground. The brake wire is the oposite in a stang. This would result in the need for a relay to change the 12V brake to ground when you depress it. I could walk you through it if your willing to get a vid of the victim.