Original head unit? If not it could be the wiring at the stereo.. you also (if its not a base model) have an amp located at the passenger footwell.. it's under the carpet where someone might have played with the wiring, and It's not impossible (if its base model) that speaker has gone out. Base models also do not have the subs behind the seats. Use the search feature here on the forum to find more than you want to know about the stereo setup.