You should be able to run an aftermarket H/U with the factory bose amp and speakers with a interface from PAC, check out
www.logjamelectronics.com and I believe what you need is about $100
Here is the part # I believe you need:
P.I.E. GM24-0S2
GM Class II Head Unit Converter
2003-2006 H2
I have thier interface for adding an auxillary amp to the factory Bose H/U and speakers.