Given that the wiring system has been messed with, I think your only option here is to get ahold of a factory wiring diagram and see if you can trace all these various and unrelated systems to one junction point, or a place where cross-wiring could have occurred. Maybe the dealer, or a good car audio shop can provide the diagram. I doubt Chilton's or Haynes would have a very good diagram, but the GM factory workshop manual might. Also try eBay, but be wary of CD manuals, which are often not very good.
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