More details, please.
What did you mean by "not working"?
Black screen? Or, screen remains the same? Or else.
It matters since it is related to the issue.
e.g.
Black screen: Reverse signal triggered, but camera sends a black screen video or no video. So, camera could be bad.
No screen change: Reverse signal did not trigger the system. Could be loose signal wire or something wrong inside the MID/system.
etc.
P.S.
The system(MID) has a R signal wire into it. This wire is high (5V) when you put gear into "R". When this wire is 5V, the MID takes in the video from backup camera and displays it on the screen. When this wire is back to 0V, the MID goes back to its previous function... i.e. whatever page you were on before putting gear into "R".