Your gonna have to do some fancy work to operate a sub in the trunk. I'm not sure it can be done, and if your planning on using the Bazzoka setup, they have a deficit in bass range. They are unique in there design, but I don't like them at all. The sub in the car also has a deficit, but enclosure space, and the idea that your sitting to close to it, makes sub freq. impossible to obtain. My thoughts on upgrading the sound system, would sound strange to most, but I've mentioned it before. Make your goal to achieve true flat responce, or may I say as close as possible, with as wide a range as possible. I'd like to hear what your plans are, but if you have not addressed achieving a flat responce in the car, all you do will most likely be a waste of money and time.
Skip