I'm doing the same thing right now with the same unit.
There should be a Light Purple (mine has a tag) wire coming out of the back of the head unit for a Reverse Signal wire, you will have to connect that wire to which ever wire your car uses for a reverse signal.
You could use the reverse lamp power wire for this, or you can try to find a wire at the front of the car that is only powered when you're in reverse.
Are you able to use the Rear View button when the car's not in reverse? You can connect the camera power to a switched wire to where you can use it anytime the car is on an
