Have a look at this site
https://www.rearviewsafety.com/My setup is wired and installed on a 2003 Silverado and a trailer. I replaced my rear view mirror with a 9" monitor using the mirror mount on the windshield. My monitor has 5 inputs. Up to 4 can be displayed at once. I have one on each side of the trailer near the front and one on the back. I also have 2 cameras on the back of the truck. One is near the tailgate release, the other one on the license plate. I have all of the cameras wired to be on if the truck is on. As long as the cameras are connected to 12v they will send a signal to the monitor. Having them connected this way allows me to view them at anytime.
I don't know if this is legal, but since my 2018 car has a switch that displays the rear camera on the rear view mirror, from the factory it probably is.