I would defo start from buying new Varta battery 
http://www.ebay.co.uk/itm/Varta-I1-V...AAAOSw5AFZZ5J-
Go to battery module and change last digit of the existing serial number to let system know that you've just fitted new battery.
Then with ignition only on manually clear faults from every single module and when you done ignition off, lock the car for a minute and fire her up.
Don't use clear all the faults option as they don't always clear.
While your old battery is out plug the amp back as it should be and then connect new battery.
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2006 D3 S8 5.2 V10
Ceramic Brakes, Bang&Olufsen, DTV, DAB, Rear Camera, BT, TPMS, Soft Close, Auto Boot, AMI , 4 Zone AC, Home Link, All Electric Blinds, Fingerprint, Ski Hatch, Black Extended Leather, Heated Sport Seats, Alacantra, Carbon Trim inc Aluminium Pack 2.
2015 A3 S line 1.4 TFSI - for the missus to keep her away from my car.
Standard S-line equipment, no extras.
2006 D3 A8 3.7 V8 - SOLD but not forgotten
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