WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Ignition I Voltage & 12-Volt Reference Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 2: Ignition II/III Voltage Schematic
Courtesy of GENERAL MOTORS CORP.
Fig 3: Ignition III Voltage & Accessory Voltage Schematic