Connect the fused end of the power wire to the battery terminal. Do NOT insert the fuse yet!!!!
Run the power wire through the grommet in the firewall. This is behind the battery close to the top corner of the hood. There are a couple of plastic shrouds that need to be removed to get to this.
The power wire should be reachable from behind the glovebox and will drop down toward the passenger side kickpanel. The kick panel and under-glovebox panel need to be popped off. The carpet has a couple nuts that hold it in place. Remove these first.
Run the power wire down the passenger side and under the rear passenger seats.
After removing all the plastic pieces in the trunk you'll see that on the passenger side there are the brains for the satellite radio and what I think is the voice recognition. I am not sure if the Navi unit sits here as well. I don't have Nav. I wanted to put the amp in this corner so I had to relocate these. You'll see their new location below.
I tapped into the existing factory headunit outputs for the signal and amp turn on signal. I got this form the Bose amp connectors. The Bose amp is on the drivers side in the trunk under the plastic panel. Pin 20 is the turn on wire. Pins 33,34,35,36 are the signal wires. I took a decent RCA cable and cut the ends off. After stripping the wires you can connect these directly to the wires coming from the pins listed above. Here is a thread that explains which wires are which, their color, and location:
About 7 posts down you'll see a PDF that has the diagram you need to identify the wires.
Once you have the proper wires tapped, run them over to the amp area and hook it up. Insert the fuse now. Fire up the radio and make sure your amp turns on. Hook up your sub and make the amp adjustments you desire.
Put all the plastic panels back on. This takes patience with all the stuff under there now.