The "backup camera trigger" wire, next to the illumination wire, needs to be wired up to the reverse wire on your car, not to 12V. Usually this is a wire found in large bundle behind the kick panel near the passenger door. With this connected, it should automatically switch to the AV input when you are in reverse. I'm not sure how you can wire it up to be enabled/disabled by choice.
I've been using the panel in my car for ~ 3 months now and it's fantastic for the price. We're now in a very cold Melbourne/Australia winter and it functions well at zero degrees, touch screen works well and the panel quality is much better than OEM units that come standard in many cars. Most people I've shown off my carputer too have been surprised for such a low-resolution display how bright and clear it looks. I'm not able to run at 800 x 480 resolution either (some other 1xxx by xxx widescreen resolution), taking this into account as well. I'd say the failure point of the unit would be the very fragile ribbon connectors. Make sure you tape the cables down to avoid vibration/damage during use. If they are loose consider putting a small box over the controller board + cables to protect it.