1996 A6 Factory Radio

I'm hoping someone on here can point me in the right direction. I've got a 96 A6 that was originally equipped with the Audi/Bose system (as
indicated by the badges on the speakers). The previous owner replaced the factory stero with an aftermarket head unit, but left the factory wiring plugs intact. The aftermarket has since failed, and I have purchased a stock head unit off of ebay (an Audi Delta). I spoke with the local audi dealer, and he told me that as long as the radio was taken from another 96-97 model (A4,A6,A8) that it should work with my existing system. Sure enough, when the system is plugged in, it powers on and says "SAFE" on the display. Here is where things get difficult. All documentation I've seen says that to enter the code, you must hold down DX and FM1/2, yet when I do that, nothing happens. The radio continues to say "SAFE", and never enters code entry mode. I have atempted to leave the radio on for an hour (with the ignition off) but still no luck. Any ideas?
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