I installed a pair of amplified 8" Bazooka brand bass tubes in my nieces 06 ion and they added a fair amount of bass without overpowering the factory speakers too much. It sounded very nice. Also, the battery is in the back so it was very easy.
The amplified bazooka tubes have speaker level inputs and a signal sensing circuit so connection to the deck was as easy as just connecting to the rear deck speakers (pretty easy) and the sensor circuit meant no other connection besides power and ground.
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