I'm using a Peripheral GMAH24B and GMAHCM to interface from an Eclipse navi headunit to the factory Bose amp in a 2004 H2 with no problems. I'm not using Onstar though so I can't comment on if there are any issues with Onstar use.
I first tried a Soundgate Digistar-B but found out that it isn't compatible with my 2004 and the work arround is to hack a Digistar, XR4 and extra speaker for chimes together. I didn't like the sound of this so went with the Peripheral interface instead. There's a thread here which details my problems:
http://elcova.com/groupee/forums?a=tpc&s=2826088551&f=6706011751&m=439104568 &r=622104868#622104868
Mark