If you plug in your camera to the 4" camera harness you purchased from us and are getting a black screen, you will want to make sure that the little pigtail with the GREEN and ORANGE looped wires (about 3 inches long) is plugged into the harness. 

This is a 4 to 6 pin molex plug adapter, it is not used in this application so let it hang or tape it up, but it has to be connected or the camera will not show.