What it sounds like to me is a short from your brake switch to your horn. What I have seen happen is your wiring loom will drop onto your steering column and rub a hole in it and complete the circuit. What is happening is your brake switch hot wire is contacting your horn wire after the "switch" aka your horn button and completing the circuit unwanted. what i would do is remove the horn fuze and if your horn still is going off this is good! All you have to do now is unplug your horn and get a test night and test at all the connectors to see where you get unwanted power and doing that you find where your short is! One thing is do not poke the wires! that causes corrosion! check at connectors and dont get frustrated. Electrical problems are hard to find but simple to fix. Mechanical problems are easy to find and hard to fix :) anything else?