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More than likely the output transistors have gone bad on the amp. There's a guy on eBay that repairs them for about $450 - several people have used him with good results.
Yes, I have the Mark Levinson option. I figured how to start the Diagnose mode. Pics are attached :
I have a 2004 gx470 and radio would not come on. I just went and hit the steering wheel while holding the mode switch and it started to work . I have had this happen several times over about a year. I think its my spiral cable but until other issues come up I am leaving it alone for now .