Visual/Physical Inspection
WARNING: This page is about a different car, the 2002 Pontiac Grand Prix. However, it is still accessible from the selected car via links, so may be relevant.
Inspect for aftermarket devices which could affect the operation of the radio/audio system. Refer to CHECKING AFTERMARKET ACCESSORIES .
Inspect those system components which are easily accessible or visible for obvious damage or conditions which could cause the symptom.