Metra makes harnesses specific to each vehicle. It will look similar to this:
The plug end plugs into the wiring harness that's already in your car, which used to plug into the back of the stock radio. The other end with the bare wires you splice into your amp's output.
The Metra harness has each wire labeled, like "left front speaker +" and "left front speaker -", which you would splice to the wires coming from your amp.