There should be a measly 15w amp under the passengers seat from the factory. It probably no longer functions, and if it does you wouldn't know it. I would look to put a small sub under the passenger seat for bass. Here is the unit I used.
Amazon.com: Kenwood KSC-SW10 5-Inch x 7-Inch 150-Watt Max Powered Subwoofer with Remote Control: Car Electronics Or you may find something better.
You can run it off the 12v that went to the factory amp. I think your biggest problem is probably the head unit though--maybe look at some new Becker or Blaupunkt units, RPD-435 maybe. You can use the 3.5mm line in to the sub for your ipod instead of the head unit as most of the ones you are going to find that look period correct won't have an aux port.