Radio Displays LOC, LOCKED Or CODE (Non-Class 2 Radio)
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: Radio Displays LOC, LOCKED Or CODE (Non-Class 2 Radio) Diagnosis (Steps 1-5)
Courtesy of GENERAL MOTORS CORP.
Fig 2: Radio Displays LOC, LOCKED Or CODE (Non-Class 2 Radio) Diagnosis (Steps 6-8)