If your running a stock headunit but already have an amp and subs, then you must be using a line out converter. Or you have an amp where you can tap into the rear speakers and get a signal that way.
You will prolly need a wire harness to the sake of making it easier. Or, you could butcher the harness as long as you know which wires do what (12V, ground, front right/left, rear right/left + - wires).
You will prolly need a wire harness to the sake of making it easier. Or, you could butcher the harness as long as you know which wires do what (12V, ground, front right/left, rear right/left + - wires).