The Dension product looks sweet, but keep in mind that without nav you actually don't have the true Audi MMI system, but a stereo that simulates many of the MMI's features. So you should check with them to see if it will work on B8s with the Symphony or Concert radios without MMI.
BTW the product on eBay is almost certain not to work if you don't have a nav-equipped car as it does require the full MMI system.
It would be purely for my wife's viewing pleasure during road trips... and the fact that we don't really enjoy the same music, so it would make for better background noise for me as well.
The screen is angled towards the driver... I think that she'll end up with a neck ache more than anything else.. (or maybe a headache). Look into something like a portable media player. Cheaper, long battery life and you can use it outside the car (and plug headphones!)
So is the E-bay unit going to work on the 2009 Audi A4 3.0L fully loaded MMI with Navigation model and a DVD Player? Has anyone tried it? I will e-mail the manufacturer and if they say okay I will buy it and report on whether it works or not.
The above should explain that by default an A4 will not have full MMI, but a radio mimicking one. You might eventually find an after-market video-in adapter that taps into your display cabling, but no official Audi solution exists. Also, Dension 500 will not work for you - it is for MMI and it won't support video display as is either.
If a car has MMI 2G, the following are relevent - but again MMI 3G is different (the new one starting from model-year 2010 in A4)...