If your rear camera is like mine, once you pull the camera loose, the wire goes through a hole into the rear cap and is sealed with silicone. I had to carefully cut the silicone loose so that I could pull out enough wire to check the connection. The wire cable on the camera was about 2-3' long so you will need to pull the wire SLOWLY out to see if the connection is loose or corroded. If it had become completely disconnected, it may be fun trying to locate the other end.
On the monitor itself, if you pull it out of the dash, you will find the wires that connect the cameras from the rear and the sides. You should be able to tell if the connection for the rear camera is loose or corroded.