I just put a sub into my 05. Right above the brake pedal slightly to the left if you take off the plastic mud cover, you'll see a rubber circle. You can break through it to wire into the battery, or you can go around it. I will try to take a picture tomorrow when its light outside.
Be careful, this is VERY close to the brake lines and other juicy stuff like that. I went around the rubber circle, duct taped the power cable to the amp to a wire coathanger and navigated through towards the battery, then removed the tape and hanger. If you go through the rubber though, it will be less likely that grime/water will get through and leak inside. The rubber is pretty thick and not easy to get through.
If you do it like this, the best place to install an aux switch is in the little cubby hole to the left of the steering wheel. I never use it anyway. Hope this helped.