I installed an aftermarket HU in my car and added a $25 ebay rearview camera. It is installed in a light housing that replaces the factory license plate light. It is not an unsightly black blox stuck on the back of the car, looks factory, and you don't notice it is there unless you look for it.
Pic of it in operation:
The camera I have is very similar to this one:
Sony CCD Nissan Tiida Livina Car Rear View Camera Back Up Reverse for GPS Radio | eBay