Keep in mind if this is an aftermarket alarm system the dealer is going to tell you to take it back to the people who sold you the alarm, not to mentions the cost to have a dealer work on it if they decide to help you. Ouch! This is NOT a difficult problem to troubleshoot. You can do it with a schematic and a DVOM. I'd be willing to atempt it without the aid of a schematic as long as I could locate the horn relay which isn't hard to do. If your alarm is putting ground on the control side of the horn relay the horn will sound. If you look at the underside of the horn relay socket you'll probably see a wire coming from your alarm soldered to the horn relay control side ground wire. Disable the alarm(pull fuse) to see if the horn stops. If it does you know the alarm is the problem. If not you have to see if the horn switch is shorted. Ford puts the horn switch inside of the airbag on some models. The horn switch can either go short or go open. You can't fix it either way if it is inside of the air bad. You have to remove the airbag(kill power) to get to the wires going to the horn switch which are located behind the airbag. You cut the wires going to the switch and solder on some leads. You drill a hole in the top of the steering column. You pass the leads you soldered onto the horn switch wires through the hole you just drilled Then attach a doorbell switch($5 at your local hardware) to the leads. Done! Either that or you can pay $800.00 for a new airbag which just happens to come equipped with a new horn switch. The choice is yours. I know which way I do this repair. Parts $5 Labor 1 hour.