I tried your approach, and may be missing something. I used a mini headphone jack, split out the wires into the R+, R-, L+, and L- then spliced them onto 26 Ga butt splices. I inserted those butt splices into the 4 individual pins on the back of my Pioneer car stereo. I suspect there is more I need to do. When I connect my MP3 (Sansa) player and try playing it through the CD, the input does not seem to be enabled. Do I not also have to enable some switch, and not sure how to do that using the pinout diag. you posted.
Any help is appreciated. I'm trying to save money and be innovative.