the sub woofer is located right in the middle of the rear of the Car. For the cars with the base stereo, If you look inside your trunk you will see an opening in the sheet metal already pre-cut where the sub woofer would normally go. It will be very easy to install an aftermarket sub woofer with the dimensions. The installer will need to cut a hole in the material inside the cabin. I would also recommend to purchase an amplifier as i dont think the base radio has enough wattage to power a sub woofer or any additional speakers for that matter. Thankfully the car battery is in the truck so an installer can easily plug an amplifier and feed it power.
The speaker layout for the three different sound systems is below so you can see the difference between the base, upgraded 400 watt system and the 900 watt HK sound system.