From what I just read, you may not need the code if it has never been enabled.
This is from my RAVE CD...
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Activating the code
Your dealer will have correctly activated the
security code before you took delivery of the
vehicle. This will be confirmed whenever you
switch the unit on by ‘CODE’ appearing BRIEFLY
in the display.
If this display is absent, the radio will still work,
but will not be protected by the security code. If
this is the case, you should activate the code by
keeping the ‘up’ tuning button depressed while
turning the set on.
NOTE: Before activating the code, ensure that
you have the Security Information card with the
security card on it for future reference.
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I have had the battery on my 99 Disco 2 disconnected a few times now and have never had to enter the code.
I think the number that the dealer is talking about is the serial number of the radio. It is stamped into the metal side of the unit, but I'm not sure if you need that, or just the VIN for the dealer to look up the code.
Hope this helps.
Dan