Detect a non-human moving object I would like to design a system in which I am able to make a light flash when a motion sensor detects a movement of a non human object such as a car. 

(The goal here being that when a car is moving behind me in my blind spot around the corner of a wall in my parking spot I don't back into it.)

I have looked into a few tutorials using arduinos to detect this motion but so far I have only read about the PIR (which only detects humans and animals).

http://www.instructables.com/id/DIY-Arduino-Motion-Sensor-Lighting-Control/?ALLSTEPS

What sensor would be best for this situation or do I need to rethink my system?

(I have no intention of harming anyone or anything :) )
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.