I have a '96 Subaru Impreza. As soon as the horn engages it blows the fuse. I've tried disconnecting the horn and the fuse still blows. The only thing I can think of in an earth leak (exposed wire touching the car body somewhere). Does anyone know where the best place to start would be, or any tricks for finding where this contact may be happening?

All right. I managed to find the problem with the wiring between the first and second horn after a bit of messing around. Firstly it took me forever to find the second horn which is located just on the passenger side (same side as first horn) in front of the radiator. It sits between the radiator and front grille. I tested both horns and found them both working so the wiring which runs from the first to second horn was the problem. I found the wiring was rubbing on the car body at a point where it passed through a hole in the body. It has worn the insulation off and it making contact and shorting the circuit. The wirings all replaced now so it's not a problem.