I haven't posted in this thread in a while, so I figure now would be a good time to:

To answer the above poster's questions, ANY aftermarket head unit will fit in your car, as long as they are single din, din and a half or double din units. Our cars can't fit anything larger than a double din (it's rare, but some companies make 2.5 din units). There is a standard mounting kit that can be used to secure it into your car.

As far as Best Buy, as long as the people know what they're doing, then yes. They should have no problem doing an install.

I ended up purchasing the steering wheel control, which is nice. It allows me 3 functions: The right wheel does volume up/down, the left wheel does track forward/back (or if I have it on random, just picks a random one), and the mode button swaps between Radio, CD, SD Card, USB, XM Radio or Auxiliary. I believe the steering wheel control was only about $80, and it worked flawlessly.

As for the rear speakers not working, I've solved this also, to a degree. The rear speakers in my car are amplified. If you have a Symphony 2 system, you probably have the same issue. The easiest way to make them work is to use the RCA outs of your after market radio and then attach those outs into the left and right speaker leads from your cars wiring harness. It's kind of ugly back there, but it does work.

From that point on, your rear speakers WILL work, but they'll be very low volume. The next thing that you need to do is shift the balance of your cars radio from the middle to almost all the way in the back. Your front speakers won't really lose anything, except you'll just raise your volume a button or two higher than you're used to doing, but your rear speakers will work and you'll be able to hear them well.

Pictures incoming.

 

Ok, I lied. Pictures aren't incoming, but I quickly made a video. Enjoy it, and I hope it somehow answers some of your questions.

It's a basic video, but it's functional.

http://www.youtube.com/watch?v=9oomrGSM-mI

 

Does the rear speaker problem occur in cars that don't have the symphony 2 system? And I take it that replacing the rear speakers with aftermarket ones would also fix the problem?

 

It shouldn't occur in those vehicles. The reason it does is because the Rears / sub on Symphony 2 vehicles are powered by an in-car amplifier. The fronts and center channel, however, are not.

Replacing the rear speakers, WOULD fix that problem in a Symphony 2 vehicle, but ONLY if you ran speaker wires from the new speakers all the way to the new head unit. Using the old wires would make your car believe that the speakers are still amplified, and they wouldn't play.

A more in depth reason as to why they wouldn't play is because amps use a common ground, with a single "signal" wire. Normal speakers use standard speaker wire, which has two connections to complete the circuit.
I hope that didn't lose anyone.

 

Anyway, here's how you see if your car has a Symphony unit. There's 2 different styles.

This is the Cassette / CD player combo


This is the CD player only version


You'll see in both pictures, the word "Symphony" printed in white letters on the Radio. If your car doesn't say that, you won't experience any rear speaker problems.

 

Awesome, thanks for all the help. One more thing though, I'm looking to buy an '06 A4 and in the description it says "premium sound system", I'm guessing that's the symphony 2? There aren't any pictures of the center console unfortunately.

 

Yeah, the premium is a Symphony system.