I did it for the stock FM/AM/CD on my 2002 X. I got the schematic from a post on this board. Its been awhile so the details are a bit fuzzy at this point, but used the 8 pin DIN connector intended for the CD changer control. Added a button to switch the head unit between AUX and normal mode. None this was my idea (like i said from this board, probably the 2000 mod you are talking about) but it works for the 2002 head unit too. Let me know if you need more data.

Right now i'm using the input for a Sirius reciever (couldn't stand the crappy FM transmiter anymore), works like a champ...

Many thanks to whoever posted the original info on the CD changer port (electroken i think you helped out, or did all the work, like I said its been awhile ).