There is a sub woofer in the rear of the car. Facing the car, it is to your right. There should be a speaker cover you can see. Replacing just the sub won't do much (may make it worse). You would need to add an amp for it to make any difference. You should also have tweeters in the front doors. Most folks just disconnected those and use the 6.5s in the doors. Good luck!
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