if you have one then it would be in is black enclosure on the side of the trunk. pretty obvious looking if it's there. not all of them (even the ses) came with one.
and changing out the sub isn't gonna sound or perform any better unless your happens to be blown.
2013 F150 4x4 SCrew XLT 5.0L
Midwest Crew #22
Official FocusFest Event Planner