mostly audi concert 2 radio's
Not without updating the wiring to include the CAN bus wires to the radio.
http://www.ibiblio.org/tkan/audi/radios/concert.html
You have to call your nearest Audi dealership and address your problem to them. Have your VIN (Vehicle Identification Number) ready. They will ask you for it. If the radio is not originally from the car, you will have to have the serial number to the radio ready. They will after tell you the code via phone.
Best bet is to go to an Audi/VW dealership, and have them reset the radio by using a VAGCOM. The other option would be to purchase a VAGCOM yourself (http://www.ross-tech.com/). Of course if you have the actual reset code, you can enter it in manually. I can't tell you specifically how to enter it on the Audi radio, since I have an Audi Navigation system, and the controls are different. But it should be a combination of using Volume and Seek (or other keys the would allow for scrolling) to enter in the code.
Your safe code for your radio is unique to the vehicle. You can go have it reset at an Audi dealership for $20-40 dollars.
The radio fits the hole but there is a MAJOR wiring change. Concert II (the CD version) runs the radio illumination and the key in (switched power) signals through the CAN + and CAN - wires on the Concert II radio. These wires do not exist on a 1999 A6. This problem can be solved however and you can get the radio to work in your car. One option is to buy a CAN Emulator. This is basically a computer chip that creates the CAN signals needed to run the radio. Another possibility is that you can run CAN wires from your instrument cluster to the Radio but this will only work if your Cluster is new enough to have a CAN chip in it which should be cars with VIN's later than XN200000. Unconfirmed but cars as early as XN061283 might have a CAN chip in the cluster.
While the radio will physically fit, and plug in the actual wiring is different. The Concert II CD uses the cars CAN bus to control things like the driver information display, on/off with key function, gala speed sensitive volume, and night time illumination of the radio face plate. Your car may be equipped with a CAN bus but there are no wires from your cars CAN bus running to the Concert I radio. In order to get the radio to work correctly you will need to run the wires your self or purchase a CAN simulator from Germany online. All other functions of the radios are the same and will work with no modification.
A combination of pulling and pushing the 2 speedo buttons at the same time. This should reset the service light.
Indicator lights on an Audi can be reset by a mechanic. The cars manual will also give instructions on how to reset this light as well.
maybe the plug under your seat has been unplugged, you will have to check the connections and then get the computer system reset using vagcom by an Audi dealership to see if that has resolved the problem
Hit the AM radio button. You did not say it was a factory radio though.