You have a short in the amp circuit. Start by checking the lines to the rear seat cigar lighter and to the horn relay. I would disconnect both, the lighter has a plug connector under the drivers side of the seat and I would disconnect all of the wires from the horn relay...be careful to insulate the ends so they don't go to ground. Then, see if the light goes off. If so, reconnect one at a time and when the light goes back on, you'll know where the issue lies. If that doesn't do the trick, then the issue may be with the ignition switch.