1997 Hardbody SE Horn issue

[WhiteKnight240]

I'm a new member here. I was recommended by a friend of mine at school, who sold me his truck, and told me this was the place to come for advice. Well, my truck seems to have developed some issues. The main one being that whenever I turn the wheel, the horn goes off, and makes a kinda machinegun horn noise. It does it even when the truck is off, and it is sitting. What could be the issue? Do I need to supply pictures? Please help, as I am trying to get it ready for sale, so I can purchase a 240. I've already posted this in the trucks forum, but to no avail. The traffic round there is kinda slow. I do not believe the horn is rusted out or anything else of the sort. The truck only has 121,xxx miles on it, and it's an SE that's fully loaded. It has an aftermarket stereo with the rear speakers replaced.

PLEASE HELP!!!