If your looking for a little more bass, then you can go a few routes. Depending on your budget, 1 10" sup and a 250W amp whould take up hardly any trunk space and give you a little more bass then what you have right now. If you absolutely cannot put anything in your trunk and are some what good with metalwork, you may be able to fit 2 8" subs where your rear speakers are right now, mount the amp on the backside of the seat. Need to know more about your budget and amount of space to work with.