P.I.E Aux Input Adapter For GM Connect a portable music player, satellite radio tuner, or DVD player to your factory GM receiver (Model GM12-AUXv2) at Crutchfield.com

That should work, but double check. Since they are out of stock you'll have to find one somewhere else, but that website can at least confirm the fit.

Before I found Crutchfield I tried doing this myself, but I eventually just bought a new stereo with aux inputs and an ipod hookup.