* Under the rear floor panel: Look for a panel or access door in the truck bed, often covered by a plastic or carpeted panel. This is the most common location.
* In a storage compartment under the rear seats: Less common, but some models might have a storage compartment under the rear seats where the jack is located along with other tools.
You should also find a spare tire, lug wrench, and owner's manual in close proximity. The owner's manual will have the most definitive answer on the precise location for your particular truck. Check your glovebox or the console for it.