You will need an amplifier. The stock six-speaker system uses four line outputs from the stereo head split into four channels (Front left, front right, rear left, rear right). Your front doors and dash corners are on the respective front channels, and your 2x rear doors on the rear ones. Best to get an amp with a separate sub output. Running power to the amp, and the speaker outputs over to it shouldn't be too hard if you've got any wiring experience. Running the wire from the amp to the sub will probably be the most challenging part. You might be able to get away with an amp that drives the sub only but I'd say just go ahead and plan for a complete system upgrade if you want it to sound good. That means add a minimum four- or six-channel amp with sub output. Upgrade your speakers while you're at it. You can use the same head unit. If you upgrade the speakers, use the Metrix-connectors (check at Crutchfield) - they come free with speaker upgrades. Crutchfield has pretty good recommendations so check them out and give them a call if you have any questions.