Circuit/System Verification
WARNING: This page is about a different car, the 2008 Cadillac XLR. However, it is still accessible from the selected car via links, so may be relevant.
Ignition ON, observe the scan tool ACC Gap Switch parameter while pressing and releasing the Mute/Seek + switches. The reading should be 2.4 volts with UP pressing Seek +, and 1.1 volts with DN pressing Mute, and inactive when released.