You should purchase a radio harness for your car, and it's wires will connect to wires of the same color from your JVC radio harness. Join the two harnesses together (solder, crimp caps, barrel connectors, etc.) and then one end plugs into your JVC, the other end plugs into the factory harness that the original radio was connected to. If for some reason, your factory harness plug has been cut, you'll have to hardwire the JVC harness. I've included a link to the wiring diagram for your car. The steering wheel controls are moot; they will not work unless you've got some aftermarket adapter that allows them to interface with the JVC radio.
2004 pontiac grand prix speaker wiring diagram Google Search
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