Honestly ive been thinking of doing the same to mine. i have the Bose system in my GT and its a pain in the ass. I want to replace the HU, but if i do i need a proper HUmount (because i refuse to use that tacky looking Scosche bull****) and i would also need the steering control adapter... too much money for now. On top of that, a proper wiring diagram for the trunk amp harness is almost impossible to find.. so i may just end up having to say fk it and end up replacing it after all. If anyone has a good diagram of the trunk wiring both me and Killroy would be appreciative. The only info i have is that the PINK wire on the harness going into the Bose amp is the remote turn on. Havnt been able to verify that myself, though.
The cheap way is a pain in the ass and the expensive way is, well, expensive.
Though you specified that you didnt want to replace the HU or anything, here are the parts that im going to use in my GP when i do mine. Maybe they will help you out if you ever decide you want to go this route.
HUmount:
Grand Prix 1992-2003 - $65.00 : Humount, Quality Mounting Brackets
Steering interface:
AXXESS® ASWC - Pontiac Grand Prix 1997-2008 Custom Radio Steering Wheel Control Interface
