If customer is not pre-wired for camera then they are a Fleet branded vehicle and will need the custom harness Greg made.
This is cheap enough to include at no cost. Ignore the default values associated to it.
G-INTCAM-17DRZ
They can use a voltmeter if applicable to identify their true power and ground wires within the power connector once they remove it from their factory digital mirror, or they can take the vehicle to a dealership who would be able to locate power and ground for them.
Alternatively the gray 16 pin connector that goes to the upper center console. Pin 1 is Ground & Pin 9 is Power. This is battery power, on all the time.