Hummer17,
I junked the OnStar. However, the easiest thing to do is to mount a different "magazine" type changer (like the one I posted the link to earier in this thread) somewhere in the truck (it comes with a 16ft cable). Then you would not have to do anything with your OnStar at all.
I have thought about a "Digital Music" connection. I wouldn't buy a specialized adapter to work with an iPod, or any other player. I don't want to get locked into a certain brand. Instead, I'd buy this adapter that gives you a standard RCA input:
SVG-707 General Motors Class-II Auxilary Input Adapter
I don't own, nor have I tested this adapter, but it is the one I would buy.
BTW - The same company that sells the AUX adapter also sells the magazine changer:
USASpec SCD-G7i 2002-Up GM 6-disc Changer