You need a on/off switch, otherwise it will drain off your car battery.

I've been thinking to setup, but as 2x single units. It depends if your car has the cigarette adapter in the trunk (rear). Then it's as simple as hiding the camera wires.

If not, the option is to tap the cigarette lighter in front, go under dashboard, down the lower A-pillar, into bottom door sill, go to rear, and solder into a cigarette adapter (Pasar Road, Pudu). Else, just cut the wire and direct into camera wire.