There are two video inputs - labelled 'camera in' and 'video in', next to the RCA connectors.
A backup camera is connected to the 'camera in' and the secondary video or a front camera is connected to the 'video in' connector.
There is a green trigger wire next to the video connectors.
The monitor will display the backup camera when the green wire is trigger with the backup light (or reverse gear).
The monitor will display the video when the green wire is NOT triggered and there IS video signal on 'video in' connector.