When I bought my wife a Lexus we found that it was extremely difficult to judge where the rear end was, so I bought a T23P reverse sensor, this is the type that has a silver strip to fit inside the plastic bumper. Three years later I fitted it, it was simple and quick to fit and works really well, it detects anything you are approaching, animal vegetable and mineral, it seems extremely reliable and far better than the ultrasonic types I have fitted for customers in the past. It goes to solid beep at about 250mm, so is quite usable with little danger to the bumpers. It can also be adjusted for sensitivity.
Mrcheerful