Buy a wiring harness - simply plugs into the stock harness plug and then gives you the wires clearly marked out. That way, you don't wreck the connector in the car, and you can swap with another HU later (or even put the stocker back in).
Besides, it's a pain in the butt to work on the wiring in the car - the harness lets you build it all up inside the house, so you just plug it into the car later.
Of course, if some idiot cut the connector already, then you probably can't use this advice. In which case, I advise searching here and at Probetalk.com as I am sure the answer is already posted.
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