If your only looking to add some bass to the system I would say a factory deck would work just fine. Is you were using a high quality four channel to run your speakers then you might run into some weak signal/poor sound quality issues.

You would have to find out what wire going to the factory deck is the 12v. Or you could run it to the stereo fuse in the car's fusebox. To run an amplifier off from your factory stereo you will need a line output converter. This will create a signal for your amplifier to amplify(Taps into the factory harness and creates 2 RCA outputs.) Another option would be to get an amplifier with high level inputs and run the speaker wires from your rear speakers to the amplifier.