If it switches to a black screen, but you dont get a picture, when you put the vehicle in reverse, then it is a camera related issue. If it does not even switch to the back-up screen, then it is a wiring or software issue with your 8400. One thing I can suggest is to make sure you have a clean connection to the backup light circuit. Try to splice this wire into the circuit inside the vehicle. Outside the vehicle if you splice it in and don't solder/heatshrink seal the connection it may oxidize the wires and lose the connection, or create enough resistance in the connection that the radio does