Remove the battery and you will see a rubber gasket that you can cut an "X" in and run you wire through the fire wall into the cabin. If you remove the pass. seat, the door still and the kick panel you can pull up the carpet and run your wire along the floor. There is a space in the bulk head near the outside of the car where you can run the wire into the cargo area. Download the repair manual (do a search) it will help you to see how to do all this.
PS: I have that Infinity sub in my pickup and it sounds good.