I'd like to install an stock Audi radio from a bone yard in my '83 5000 from a '85 from a bone yard. The unit has a red key graphic on the face plate. I assume this is a theft deterrent. Will this work? Do I need to also get something from the '85 to make it work? Or would it be easier to just go to my local stereo shop and buy a new unit? thanks Jeff Petersen