I purchased individual components (monitor, camera's, and cabling) on eBay, to make a multiuse wired system. The monitor clips on over my rearview mirror. I wired the truck so the monitor plugs into it's video / power connector, right behind the rearview mirror. To power the system, I spliced in a 12 volt plug to plug into an outlet on the dash.
The power / video cable ends by my tailgate, where I can plug in whichever camera, I want to use.
I mounted cameras on my travel trailer, truck camper, and on a piece of metal (with a magnet) that either hooks onto my license plate for hitching up a trailer, or, I can set it on the truck bed rail for loading the truck camper.
With TC loaded up, or, with TT hitched up, looking behind me, is simple as looking at my rearview mirror. The system helps with loading, hitching, and driving. When passing, or being passed, vehicles appearing in my side mirrors, may look like they are beside the trailer, but, if I can see their headlights on the monitor, I know they are behind it.
Many backup systems, recommend wiring into the backup lights on the tow vehicle. That only gives you video when backing up. Powering from the in-dash power outlet (or any full time hot system), the camera can be on, anytime the truck is running.
Links to similar components:
MonitorCameraCableCables can be purchased in various lengths. I used one about 25' long to wire the truck, one about 50' long to wire the travel trailer, and one about 12' long for the truck camper. No extra cable was needed for the third camera.