I've been in electronics a very long time and spent many years fixing the most difficult intermittent problems on vehicles. Many time the short was in a different source than where the symptoms are seen. Where everything uses a common ground, you can get back feeding of power thru the ground. You need to be very open minded. Even corroded connections can cause alternate paths thru a device such as a horn sounding the horn. After all, most DC devices don't care about polarity, onmly the potential difference in voltage.
One of my best /worst was a 2 year old Firebird in the late 70's. Every 6 months or so, the main harness would fry. It was at a friend's shop. I took the interior apart and found that several wires were pinched in a metal dash support when the car was assembled. It only burned the harness under the hood, not the inside part.
Good luck and keep us informed.
PS I used to enjoy these situations, except when it's your own vehicle.