Are you talking 2 different monitors? The radio and another monitor? If not, how can you see the backup camera on all the time AND have a "Camera 'A' no signal" message? Unless you mean the radio is selected to see the backup camera all the time.
The backup camera is always on so that you can view it going down the road when you press the "Mode" button on the Axerra radio. You go back to normal radio operation by also pressing and holding the "Mode" button.
If you are not getting a display signal from the backup camera when you expect to get one it sounds like maybe there is a loose connection at the back of the radio (common problem, but then you wouldn't get side camera views either when the turn signal was on), or if the side cameras work OK then maybe a bad power connection to the rear camera.
Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad