WARNING: This page is about a different car, the 2007 Cadillac SRX. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Power, Ground, And Key Word 2000 Serial Data Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 2: Low Speed GMLAN Serial Data (1 Of 2) Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 3: Low Speed GMLAN Serial Data (2 Of 2) Schematic