Hi kacyk,
I used a Peripheral GMAH24B and GMAHCM to interface my Eclipse headunit to the factory Bose amp and speakers. I first tried a Soundgate DigistarB interface but it turned out that it wasn't compatible with my '04. See this thread for details:
http://elcova.com/groupee/forums?a=tpc&s=2826088551&f=6706011751&m=439104568 . It looks like you have a '03 so you should be able to use SoundGate DigistarB, or Peripheral GMAH24B (GMAHCM is not required for a '03), or a PAC unit.
The advantage of using the factory amp (rather than powering the speakers directly from the new headunit) is that OnStar and Chimes are retained, the headunit will run cooler and installation is simplified.
Mark.