I have the above in my b6 but the CD keeps throwing error up, sometimes it will play but mostly not, if i get another unit is it a simple swap job or any coding needed .
Assuming we are talking about replacing a single din Concert unit and it is not a BOSE (normally identified by the speaker grill badge) or similar system pretty much any stereo will work, you will need a fascia adapter/trim plate
http://www.ebay.co.uk/itm/Audi-A4-Panel-Plate-Fascia-Facia-Trim-Surround-Adaptor-Car-Stereo-Radio-/250790289905?fits=Car+Make:Audi|Model:A4&hash=item3a644421f1:g:xqYAAOSw-zxWn8Wt
and maybe a harness adapter, that said anything along these lines will work
http://www.halfords.com/webapp/wcs/...earch&storeId=10001&catalogId=10151&langId=-1
One thing to be careful off is the way the Audi is wired for the stereo to carry on running when you turn the key off means that if you do not use a good quality harness adapter it will cause the central locking to stop working. Also regards the wiring again there is something in the back of my head about diagnostic equipment and aftermarket radio - search the Ross Tech site regards this problem