The problem arises when contacts from one source become common to other sources in a different supply. Once this happens things get jumped out. So....Say a doorlock in the 79-00 circuit goes common with a selector contact in the 311-312 circuit only when the car is at the basement. Which is where you shut the other car off at! Because you have other relays and contacts in these circuits you won't blow a fuse, but you could jump out a safety device like a doorlock feeding 311 that is grounded but not in the doorlock circuit through a contact or contacts to the 41 relay to the other side to 00. Allowing the car to run with the doors open....It could happen! I would bet if you just could turn the car that is shutdown on...This would go away....Use the lightbulb to discover your grounds and life will return to normal!