Great project to add your MP3 player to your car (Cool MAKE Photo too!) “Over the past couple of years I have upgraded my Pioneer Car CD changer to meet my needs. I went from a 6CD to a 12CD and now I have a portable MP3 player I want to connect. Pioneer offers an auxiliary adapter (CDRB-10) for ~$40 or an iPod adapter (CD-IB100) for ~$130. The auxiliary adapter appears to be a simple input-to-RCA-out, so I decided to make an aux adapter myself for ~$6. A quick tally of the cost of the parts listed is ~$12 bucks but you’ll have plenty of wire for your future projects and an extra stereo jack“. Link.
DIY Pioneer Aux Input (MP3 to Car)