I can help you out. Your going to want to buy what is called an Inline Converter. It will splice into the rear deck spears and pull the rca's off of them.
You can get them for like $50ish or a cheap one on ebay. But be fair warned the cheaper the product the crapier the qaulity of the sound is going to be. If your wanting really clean sounding subs or speakers a converter is not the correct route.
As for hooking up the rest of the system its just like normal. Add the Power Wire, Ground the amp, run a remote wire either to a switch or buy a fuse tap and you can run it to the Sun roof Fuse if your cobalt is an SS or an LT because that fuse is only "hot" when the acc power is on.