You can also wiggle the wires coming from the amp under the back seat. I've done this with the radio on and discovered loose connections to some speakers.

With the engine off and key out, pull off volume knob and spray contact cleaner all around inside. Then do your 248 turns with everything off and see what happens.

Sadly, more than likely an amp in the head unit is gone. From roundel, there is a repair company out of Florida that rebuilds your head unit

www.soundrepair.com

I bought another one on tha bay and it cured everything, so I am going to send my fritzed one to these guys to have a backup.

HTH

geoff