The kill switch. Worth replacing with better (or new) wire as it is notoriously prone to insulation rub-through and shorting to ground.
Adding a third wire is entirely up to you, I used black so I would not mix it with the others. I cut the shutdown wire that goes to the on/off switch and replaced it with this third (black) wire.
Adding a third wire is entirely up to you, I used black so I would not mix it with the others. I cut the shutdown wire that goes to the on/off switch and replaced it with this third (black) wire.