You can use the normal aftermarket stereo kits from METRA or similar for the brackets and the wiring
For the steering wheel interface you'll need a PAC Audio SWI-PS or an AXXESS ASWC. Same installation for both, and both can have the buttons custom mapped, but the AXXESS already has a good configuration built in and it auto-detects.
For audio you can go one of two routes.
1. Wire from the speaker output of the HU as you would with a normal non-amplified system. This is not the best solution as you would never want to turn the volume past around 1/2 unless you feel like frying the amplifier.
2. Wire the line-out from the HU (if it's 4+ channel line-out) to the same wire as mentioned before. This is better because the BOSE amplifier relies on low-level input from the factory head unit.
IIRC for On-Star, you need a GMOS adapter, but having never messed with a GP with On-Star I'm not 100% sure.
Read the Audio Knowledge Sticky for more info!!! That's why it's there. Good luck with the project!
