WARNING: This page is about a different car, the 2002 GMC Safari and 2002 Chevrolet Astro. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Reduced Volume - One Or More Speakers Diagnosis (Steps 1-5)
Courtesy of GENERAL MOTORS CORP.
Fig 2: Reduced Volume - One Or More Speakers Diagnosis (Steps 6-11)