Get a line out converter, you can get this at any car audio shop. In fact you can get one at Wal-mart as well or even Best Buy. It uses the factory wiring to your rear speakers for a signal and converts it to RCA so you can hook up an amp. Depending on if you have a factory amp or not you will have to splice the LOC before the amp but keep the speaker connected (SPEAKER, as in only one) to the amp. You can do both but does not require it. All the LOC needs is a signal coming from the HU to tell the amp what frequencies to play.
Get a Amplifier wiring kit, you can also purchase this at Wal-Mart/Best Buy/local CA shop. Then run the power wire from the battery to the amp. Run the ground from under the car using one of the bolts that holds the rear bumper to the frame on the RIGHT side. Make sure you sand down the paint on the frame and any that may be on the bolt to get a good ground then paint over it once it is connected to prevent any kind of corrosion. Then run it up through the grommet on the hatch floor between the wheel well and the right rear tail light and then to the amp.
HOWEVER, since those converters don't provide a remote turn-on lead for your amp you'll have to run a switched 12v wire to turn the amp on and off using the remote line that came with your amplifier wiring kit. You can either splice it from the fusebox, from behind the factory radio or just a wire from an unswitched 12v source then put a simple toggle switch in between. I suggest from behind the radio that way you do not have to deal with a switch for there would already be one in place.
Keep in mind, when YOU do this you will then need to turn the bass on the factory HU ALLLLL the way DOWN then adjust the gain at the amp to the level at which you want it. If you do not do this when you turn the radio up it will sound like ass.
I think thats all you should need to know. any other questions feel free.
i will follow up with you tomorrow evening sometime.
