How to mod the radio
Here is the link to it.
This document describes how to add an AUX input to the factory radio. This can be phono stereo 1/8 jack or RCA type. The input will be for a "line level" signal but a signal from the headphone jack of your player will work, just be careful with the volume level. Adding this input uses part of the circuits and connectors used for the external CD changer. After this modification the external CD changer cannot be used (or installed). The modification is reversible.
Seth describes a much simple way to modify the radio on this link:
During the development of my installation I tried to do what Seth did but it did not work for me. I do not know why. An important difference is that he is using the headphone output and i'm using the line level output. This is an open issue and my suggestion is to follow what Seth did and if it works for you, just stop there. If it does not work or you want to keep the onStar/Telephone interface working then you need to go all the way.