if you already own a cd changer and don't mind hacking it up then try what i did. kenwood aux in page
if you want to build one then i think it deals with putting a resister between two of the pins in the 13 pin cable, which ones i have no clue, sorry. If your headunit is old then talk on the board is that it NEEDS to talk to the cd changer FIRST before the aux in channel can work. If your headunit is a new style then i think you could come up with a homemade cable that people had heard about but no links have ever been supplied. Maybe that $25 cable's not lookin that bad right now eh....... might save you a lot of time. Also try the search button and type in "Kenwood Aux In".
Good luck, and welcome to the board.
GTI - Palm IIIe controlled with Radio faceplate hack for keyboard control of WinAmp w/ Vocal Genre playlist loading.
VW Bus - 10.4" vga lcd. Loaded with same software as above.