The reverse trigger wire is usually connected to the backup light switch on the transmission. This connection allows a signal to be sent from the vehicle when it's in reverse, which triggers an accessory such as a camera or other device. To connect the reverse trigger wire, locate the backup light switch and follow its wiring harness until you find an open terminal that can accept 12 volts of power. Connect your reverse trigger wire onto this terminal and secure with a nut or bolt. Be sure to also use electrical tape to ensure proper insulation between any metal parts so as not to short out any circuits. Once everything is connected properly, start up your vehicle and make sure that your accessory turns on when you shift into reverse mode!