There is an alarm module that controls the horn when the system has been activated it sounds like it is going bad. If you want to cut the wire to stop the horn here is a wiring diagrams to help you see which wire to cut.
Check out the diagrams (Below)
Please let us know if you need anything else to get the problem fixed.
Images (Click to enlarge)
Tuesday, December 5th, 2017 AT 4:35 PM