Does anyone know where the rear amp for the rear door speakers and subwoofer is located at and how to get to it? I'd like to open up the rear corner panel but afraid break anything.

These parts are in the drivers side rear wagon pannel. Unless you really know what you are doing I would not attempt to get to them. I just had the stock subwoofer removed and replaced with a "kicker PS60" subwoofer. The highly trained professional installer took two hours, two calls to Audi and three calls to Bose to properly access the pannel and even then he ended up not getting it back together right and lost the seat tie down clip. Good Luck. PS. The system is greatly improved with the stock sub replaced.