In the early days of factory backup cameras, many manufacturers including Ford and Toyota began implementing rear view mirror screen based camera systems. As these cars did not come with factory navigation and large LCD screens, the monitor on the mirror was a cost effective way to add a seamless display in the car. As an aftermarket upgrade, the rear view mirror backup camera still remains popular as aftermarket radio replacement will always have a much higher cost.
Radio replacement with a basic bluetooth double din and a backup camera for a car like this would start at around $700 installed while this sort of rear view mirror camera is about half that. The screen is only a 4.3" screen and is not as clear as what a double din radio would show but it also keeps the interior looking relatively stock. We used a license plate frame backup camera which keeps a low profile and doesn't protrude past the bumper.