I fitted a double head head unit (Chinese ebay purchase for £58 included the head unit, and a wired reversing camera, the head unit does satnav, BT, FM Radio, Micro SD and USB Pen drive sockets for pictures, films and music)
The camera is currently mounted in a custom designed and 3D printed mount which sits on top of the tail gate trim with the camera looking out of the tail gate glass just under the spoiler
The wires is routed through the gator between the head lining/ D pillar and the tailgate on the LH side, it picks power up from the reversing light on the LH light cluster and the cable then goes over the rear wheel arch and along the sill, behind the clove box to the head unit. a Couple of hours tops to install it
As soon as revers is engaged the camera is powered, this sets the head unit to rear view camera automatically and mutes the audio, you then get a crystal clear view of the rear of the car and the guide lines green/yellow/red for distance judging
I can back up to another car such that I get that close that the tail gate will not open on the A2 but I never hit the other car, the only issue was rain on the tail gate, but a application of rainX sorted that
I have to say that the camera and head unit have been excellent quality and once fitted in the A2 give as near an OEM look as it is possible to get, it just looks like the RNS-H that Audi have not yet built
there are some pictures of it on the A2OC forum, from around Feb 2017
Cheers,