Probably the easiest way you could add some extra sound is to add a sub and amp, but use the wires off the sub part of the rear speakers. So what you need to do is look in your boot at the bottom of your rear speakers, you should see 2 cables into each speaker( red and black i think) you cut these off the speakers, hook up a line level convertor to these cables and then hook these RCA's up to your amp, and then add a big dirty sub for that much needed bass, you can then adjust the high,mid and low settings on your factory headunit so you can turn up the volume more without the speakers distorting as much, you can get cheaper versions of the convertor, the link i gave is just the first one i could find, think this might be easier than adding to RCA's to the factory amp if your not too electrical savvy!