I can not determine from the linked monitor listing if the monitor can be switched from video 1 to video 2 input simply by pushing one of the buttons on the monitor, but this is a feature that you will want.
The rca connectors on these things for video input are fairly generic, but one input to the monitor is normally a male with the second being a female type. If you are doing the mix and and max between various vendors/manufacturers, a simple rca gender changer plug designed for audio will let you connnect the mismatched cables together. One tip: after connecting the rca connectors, wrap with electrical tape to prevent disconnection due to vibration.
If you can stand a larger mirror, you can get a bigger display with the larger 7 inch display type.
Search ebay for "mirror mount rear view monitor" to see many options.
Once you install your system, you'll wonder why you didn't do it sooner. Being able to see behind you is very helpful.
Fred