I went with this:
Precision Interface Electronics GM12-AUXv2 Connect a portable music player, satellite radio tuner, or DVD player to your factory GM receiver at Crutchfield.com
I don't have XM or a CD changer so it worked out perfectly, and the car even reads it as Aux In (it takes up 2 bands though, replacing XM1 and XM2). Didn't take too long to install either, and I just used some 3M double sided adhesive to stick it to the inside wall where the stereo is. I also got an RCA to 3.5mm adapter and a retractable 3.5mm audio cable and drilled a small hole in the back of the little storage compartment. All in all took like 45 minutes, but really shouldn't take more than 20 (most of my time was wasted on being overly careful not to break things).
