I installed a backup camera in the fall, and I used a rearview mirror monitor. The first problem was the wireless receiver would flash because it was picking other 2.4 Ghz signals which would quickly flash the monitor to the on signal then off. I corrected this by running the 12 volts from the backup lights to the monitor,so much for the wireless.The unit worked great but then when the temperatures hit the -30 range it stopped. Naturally I suspected a wiring problem, but I had to wait months for the weather to warmup. I thought it was the mirror monitor so I removed the overhead console and checked, power was OK , so I tried another monitor and still nothing. I checked the camera and it was getting power so I just plugged in a RCA video cable from the camera to the monitor, still nothing.
Then I parked my car close to my other car, which had the same system, set both parking brakes and put both in R. Guess what the signal from the car was picked up by the SUV monitor. So it was a bad camera all the time.
So if you have a wireless system and the video stops, see if you can pickup a signal from another wireless camera system. Most are on 2.4 Ghz and if you sit in a crowded parking lot with your car in reverse you should pickup someones video. 50 years ago in radar school they said do the easy stuff first and I forgot that rule, Don;t rip it all apart, do the simple thing.