a subwoffer is esentially a bigspeaker to amplify bass.
Don't that's stupid
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check you AUX cables and the wiering to your subwoffer and amp make sure power is flowing. If nothing is good somthing is blown or burn. replace.
Yes you can... if you still have a factory head unit then you will need what is called an RF modulator which you will connect to one of the speakers. This will give you the Av imputs for your amp.
it depends what your looking for, word has been going around that the new kicker 18 solo X is the loudest with 5000watt RMS to 10000 RMS, defs like the Hifonics subs RE and the Steve Meade subs. brand new 2010!
Subwoffers come in sizes from 8 inches to 15 inches. You should try finding an enclosed sub woofer so that you won't have to build a large enclosure and you will already be able to tell of it will fit in your car. An amplifier lays down flatter so this is not as much an issue.