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Cleaning up other people’s mess is the worst. It’s very possible that they had an amp for the speakers and just took it out. One thing you could look for is to see how many sets of RCA wires were ran to the aftermarket amp area. If just one set, then I would think they probably only had a bass amp. But if two sets of rcas or more than they definitely had an amp for the speakers, and when they took it out, you lost all audio to the speakers.I bought this truck with lots of problems that I'm working through one at a time. The deck was in it but I never heard it make sound. The PO made a mess of some things. There are wires for an aftermarket sub amp, but I still suspect you might be right. I thought about ordering a new interface harness with instructions and just redoing it cleanly, but they're kind of expensive. I need to do some diagnosis to make sure that'll even work.
Could that plug have anything to do with Onstar?