camera needs + and -.
- (or ground) of the camera gets tied to the car chassis.
+ (positive) comes either from the backup lights or a constant 12 volts.
To determine which wire of the backup light is positive, remove the bulb and use a volt meter when you have it in reverse.
For a constant voltage source I'd tie it to the accessory line that goes to the radio. That way it's only on when the car is on.
So the camera is always grounded, and the + comes from either the backup lights or radio accessory wire.
Hope that helps.