If the drawers are in the unit, remove them and set then aside before installation.
With help from a friend, set the completed box on the tailgate, lift it into the truck and slide it halfway in.
Use drywall screws to attach the wing boxes to the main box, and push the entire unit into place. Use galvanized L-brackets to screw the unit to the truck bed.
Rub paste wax along the runners on the bottoms of the drawers and slide the drawers into the storage unit (Image 1).
Screw small metal brackets to the ends of the vertical dividers so they overlap the drawers slightly. This will keep the drawers from sliding out when the tailgate is down; rotate the brackets out of the way to open the drawers (Image 2).
To help weatherproof the drawer unit, cut a wide strip of rubber liner to cover the drawers completely and overlap the top of the unit by about 1 foot.
Position the rubber liner and nail it to the unit using roofing nails. Drive a nail about every 6 inches. The liner will keep water and debris off the drawers, but will flip up for easy access. Lay a pre-purchased truck bed liner on top of the box. Choose a liner that covers the entire unit.
Note: Make sure the rubber liner is in the "down" position over the drawers before putting the truck bed liner in place. Otherwise, it could be trapped under the purchased liner.
Use L-brackets to attach the purchased truck bed liner to the truck.