The serial for Audi concert radios is located on the back of the unit, this means removing the radio from the dash is needed to gain access to the radios label.
To do this is easy, all you need is a set of radio release keys, these inexpensive tools are available in most car parts stores or online at Amazon.
Press the 4 keys into place, as you apply pressure you will feel the radio free from the dash, simply pull the radio forwards (do not remove any wires from the back) the serial will be similar to AUZ1Z1F6412082. Please see image for reference.
*Free Audi Concert Radio Code instructions are also sent after purchase