Found this in the service manual (while looking to see if my SCV doesn't work by design or fault). There is a loudspeaker test mode available on the Rockford Fosgate CD sound system. (The service manual was slightly incorrect. My vehicle is USDM GSR with SSS.)

With the vehicle in ACC or ON, turn the radio OFF.

Press these buttons in order (within a minute):
channel 1
seek back ( |<< )
seek forward ( >>| )
channel 6

Use channel 6 button to switch between speakers.

The manual mentions a "buzz mode" if a CD is inserted at this point. I didn't test it.