Like bear said, underside or back of the seats / bench is one way.
Behind the door panels (if there are some) is another, inside the dash behind the glove box is one more.
If there is a carpet, there could be one underneath, but mostly it´s rotted there...
Good Luck !
Found mine behind the door panels on a 1972 D100....
I've never found one in a Sweptline era truck. The data-plate on the driver's door jamb gives you the drivetrain/suspension info, and any special equipment.