I recently bought a used 2001 Audi A4 2.8 Quattro with the Bose Symphony system. Great car, love it so far, but of course there are a few minor things wrong with it for being a 12 year old car. One of these things is my rear deck speaker on the driver's side, which is busted.
I took it to my sound guy and he said he didn't have any speakers that would work with the Bose Symphony. They are Bose 6.5" (165mm) speakers. I can only find a couple OEM speakers online.
My question is: What speakers would work in my A4? Is there something special about the Bose Symphony?

Any and all information helps, even if it's not even related to the speakers. Thanks guys