My Dad has this kind of system but I don't know if he has an amp. Usually they say it is located under the seat. He gets a really loud pop sometimes when he turns his off - did you ever get that? I was gonna say maybe the speakers are bad or the speaker wire is bad but if they're all out...I don't know. Here are some websites I occasionally use for car audio info. They should be more useful than this site. There are alot of experts that may be able to help you.
I found the amp, it is hidden behind the globe box. Rather than spending more time to figure out if the problem is the amp or stereo, I still think it's the amp because the headphones work when the stereo is on, I've order JBL amp bypass wires for my new CD.
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