So my truck has the SR5 package, and my question really pertains to the factory subwoofer behind the passenger seat.. My truck when I bought it had an aftermarket stereo already and the previous owner also replaced the door speakers.. But my problem is that the sub is not working? I have too much highs & mids but no bass... Any one know if you have to have the factory amp hooked up or did whomever installed the replacement radio just not hook it up... So I guess my real question is can the factory be used with an after market head unit? Thanks in advance..