Detect a non-human moving object
If your car is moving at the time, then relative to your car everything is moving.

Automotive companies have spent billions of dollars working on this problem.  There is no "sensor" that can differentiate one moving object from a background that is also moving.

All existing automotive systems that perform this task use cameras and computers, with software such as OpenCV.  This is not something that any Arduino could even come close to doing.

The closest you could get is "is there something in this area that is within a certain distance of me" which you may be able to do with an ultrasound "Ping" sensor.  It won't differentiate between a moving car or a bollard or a child or just the wall you are reversing towards.