The only time my horn has gone off is when the alarm is turned on from inside the car in the door lock panel by accident ,as I opened the door from inside and the alarm went off, and the only way to turn it off is to get out of my car and reopen the door lock with the door key("88 have two keys, one for ignition with coded pellet and one for the door).It all comes down to the alarm being set and me open the door.I don't know if this is same problem as you are having, just my 2 cents.