WARNING: This page is about a different car, the 2003 Cadillac CTS. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Diagnosis Procedure - No Global Positioning System Reception (Steps 1 To 6)
Courtesy of GENERAL MOTORS CORP.
Fig 2: Diagnosis Procedure - No Global Positioning System Reception (Steps 7 To 9)