How to fit a RD3 player with different code to work with anti-theft device?

I own a 307 peugot year 2003. Twice my cd player has broken down. Had this repaired at the Peugot Dealers under warranty. Has since broken down again. I have been able to purchase a secondhand RD3 player, although unfortunately did not come with the code. No problem I thought, will obtain through the internet or similar. Since digging around for information of the code number have been informed that this new cd player will not be compatiable with my vehicle. I was quoted something about the electronic management system would not marry from the cd player to my car. This is an anti-theft device.
Has anyone heard of this, or better still have they got the answer of how to get around this situation. All I want is a cd player again!
Many thanks guys for any information will be much appreciated.