To remove the original head unit you will need either the special tools (2) that fit into the holes at each corner of the faceplate, or you'll have to make a substitute. The tools can be either borrowed or purchased from most car audio places. I've found that two pieces of wire coat hanger work just as well. Bend each of the two pieces into a "U" shape such that the legs of the "U" are spaced approximately as far apart as the holes on one side of the head unit. Insert a "U" in each side far enough to trip the locking tabs (you'll feel it), then pull the unit straight out by inserting your fingers into the cassette door. Instead of coat hanger wire, I've heard people use 4 appropriately sized nails or small screwdrivers.
Places like Crutchfield probably have the correct adapter to match an aftermarket head unit with the stock wiring harness. Some people like to have a switched power lead to the head unit rather than constant power. To achieve this, you can tap the ignition lead that will provide power when the key is turned on and will continue to supply power until the key is completely removed from the ignition lock cylinder. The easiest place to connect to this power is at the seatbelt warning control unit on the auxiliary relay panel in the driver's footwell. There should be a red wire going into terminal 85 on that control unit (relay position #2, second from left on middle row). Just tap the wire for your head unit's power into this line.
There is an amp piggybacked to the frame of each rear main speaker. You can either feed pre-amp (line level) outputs from the new head unit to them, or you can bypass the built-in amp. If you intend to retain the use of the built-in amps on the rear speakers, not only will you have to feed them pre-amp output, but you'll also have to feed power to them. The black connector on the stock head unit has two white wires coming out of the same pin. These wires feed power to the rear amps and the antenna booster. Connect these wires to the "+12V remote send" output from the new head unit, and your rear amps will function, as will the stock antenna, and even the door-open chime.
(Edward Pants) If your want to bypass the amps on the rear speakers, you'll have to modify them slightly. On each rear speaker, there is a jumper wire on its little input connection panel. (There are three connectors: biggest is power, ground & signal; middle is the amp enable jumper; and third is two white wires feeding the tweeter direct -- use this connection also to feed the speaker with an external amp after removing the middle jumper). Removing this wire will disable the amps, and allow the speakers to be run by an external amp. However, the Audi Speaker amps are equalised, and you'll find out how crappy these speakers actually are when driven by a clean, straight amp.
If after installing an aftermarket head unit, you are having problems with rear speaker distortion, heavy rear-bias requiring extreme forward fader, or your rear speakers cut out after a short period of time, make sure that the rears are either connected to a low-level output, or that the amp has been bypassed if they are using amplified output from the head.
Keep in mind that many aftermarket head units do not offer red or orange illumination. If that's a concern for you, you'll be able to narrow your choices quickly by only considering the ones that do.
