Aux cable option to car stereo
Based on the service manual for your KDC-MP3029, the only real option you have to hack in an aux cord or connector is less than stellar option of soldering to the CD Player's audio output. On the CD board, there is a test point for Lch (Blue), Rch (Red), and Audio Gnd (Orange), right by Connector CN1.



Or you could try on the main board, by CN1, and C1 and C2.



This is a very hacky type hack. You'll need to burn a cd to have no audio, a 90 minute cd of silence or whatever for the audio to be heard from your aux cable. See https://www.youtube.com/watch?v=hgqa0pbc650 for an example.

Alternatively, you could try to hack in the external CD/Aux connector J2 in, but all the support parts, resistors, diodes, are not populated on your player.