Hi Melanie, How are you? I have the same car as you with similar upgrade that you wanted. I can tell you that the head unit has six speakers out put, four at the front and two at the rear. The power out put is not that powerful, 50 watts max per channel max. It depend on what you want, but If you want to keep it stock as possible then just change the two rear speakers and add in an extra sub, that would give you plenty of power and bass. I would not go larger than 10 inch for the sub and it takes up a lot of room and require more power amps. To run an amp in your car you first needs to convert the rear speaker signal coming out from the head unit into RCA signal and that can be done with a RCA signal converter. You will need two of it. They are not expensive and most audio shop will have them. Once you have the RCA signal, buy a RCA splitter and a four channels amp, two RCA for the speakers and two bridged down for the sub. I know this may sound technical and I dont expect you to understand what im saying nor for you to go and do all this on your car but i want you to at least understand that it should not cost an arm or leg to have this upgrade. It's really quick and easy for any audio shop to install. I hope this help in some way. All the best.