WARNING: This page is about a different car, the 2006 Chevrolet Monte Carlo and 2006 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Power, Ground, and Low Speed GMLAN Serial Data - Wiring Diagram
Courtesy of GENERAL MOTORS CORP.
Fig 2: High Speed GMLAN Serial Data - Wiring Diagram