Thanks! Then I'd say the mixed answers reflect two different equipments :
-Rear parking sensors (those gadgets on Amazon) - just reflect the distance between the back of the car and a fixed obstacle right behind it. About 10' range, no side perception. And even if it had it, too short for reaction. Much like the camera.
-Rear cross traffic alert - detect moving objects. About 300' range with back and side perception. Minimal time/distance for reaction ( some with auto-braking function), provided that the "view" is clear. About 40% efficiency, according to AAA.
That's the type I'm not aware of an aftermarket option beside the Mobileye, and even that one is rather a forward collision equipment.