If you dont have one, its not wortch it to get a stock one. You will need the stock amp, possibly wiring, the sub and box, and the rear factory panel. not to mention you may need the upgraded audiophile radio.
It would be alot easier to add an aftermarket one to the existing stock radio if you want. you can have a audio shop build you a custom fiberglass box for the sub to look stock and be in the stock location. It will sound a lot better also.