As with most Audi models, the removal of the A3's stereo is relatively simple, requiring only a cheap, special tool and about five minutes. Stereo removal may be necessary to hook up aftermarket CD changers and iPod docks, or to replace the head unit. The head unit in the Audi A3 is specifically designed to deter and prevent theft (hence the removal tool), and requires a special unlock code upon reinstallation.
Insert Audi stereo removal keys (longest side, bevelled end inward) in the two slots near the bottom right and left corners of the head unit.
Push the keys gently but firmly until they are fully in.
Pull gently but firmly on the keys to remove the headunit.
Push the silver clips on the side of the head unit in order to release the keys.
Unplug rear connectors by gently squeezing the coloured, plastic connecting tips (in the same way you would to remove a medicine or mouthwash bottle top) and pulling outward.
Your Audi dealer can supply the keys for the head unit removal.
Pull the head unit out slowly; the wires behind can be snagged. Upon reinsertion of the head unit, the stereo will be inoperable until you enter the unlock code. You may need to call a dealer and prove ownership of your car to get the code, if you don't already have it.