Depending on your camera. They come with 4 or 3 wires (the ground is common). The HU needs +12 to the wire labeled "back" to switch the input to the camera. Do not use the video input for the backup camera, use the right RCA connector.
If the camera is not powered or there is no signal on that input you will get a message on the screen about that.
If you don't send +12v to the "back" wire your HU will not switch the input.