All speakers need some sort of power source. You will need an amp for the subwoofer.

There are different types of subwoofers. You have subs that requires sealed enclosures, some that require an infinite baffle, and other that will require some sort of free air environment.

You can mount the a sub on the rear deck just fine. You just have to make sure that the air space in front of the woofer is completely isolated from the air space behind the subwoofer. THen you will have yourself an infinite baffle environment for the sub. Some things you might run into...make sure you have enough space behind the sub and be aware that your rear deck will flex and make noise.