aftermarket Navigation

Im looking into adding an indash navigation to my 07 yukon. I have been looking for the wiring harness online but the cheapest i can find is 130.00 because I guess I need a special harness since I have the bose system and I have Onstar. Does anyonw know where I can get a good deal of the harness? I am also looking for the adapter needed to keep my steering wheel controls operational. Any help would be appreciated.
Thanks

its expensive, I spent 130 for the harness from pac(os3), 60 for steering wheel controls from pac(swi-jack)