The red wire is used for remote turn on for the radio. It needs to be wired to a +12V ignition switch. This will turn the radio on and off with the ignition or via an accessory switch. This does not need to be connected, however, many radios will not power back on after a power cycle without it. The driver may have to turn the radio back on manually whenever it is not connected. We recommend connecting it when you have a radio that has an on/off push-button.