Today we installed a Ryden Back-up Camera into a brand new Audi S4. This is a very discreet and small camera mounted above the license plate on the trunk. This unit is tied into the factory navigation screen and turns on automatically when the car is put into reverse.
This picture shows how small the back-up camera is. It is located just to the left of the right plate mounting screw.
From a few steps back the camera is almost unnoticeable.