To install a new bed on a used Dodge pickup, disconnect the negative battery cable, lift the rear of the truck, remove the gas cap and filler neck, disconnect the wire for the tail lights, and remove the old bed. Next, put on the replacement bed in reverse order. The bed is heavy, so enlist a couple of helpers to complete the installation.
Jack up the rear of the truck, and place appropriate jack stands under the frame. Once you have disconnected the electrical supply, unscrew the gas cap, and remove the three screws holding the filler neck to the truck bed. Lift the filler neck free from the truck, and put it, the cap and screws in a safe place.
The tail light wires are connected by a harness plug that is under the back end of the truck or behind the tail light lens cover. Locate the connection for both lights, and unplug them. Slide under the truck, and remove the mounting bolts for the bed. Mid-size trucks have six bolts, and full-size models have eight. Spray some penetrating oil on the bolts to free them if they won't turn.
Use a pry bar to loosen the bed from the frame, and then jack the rear end up, remove the jack stands, and lower the truck. When it is firmly on the floor, have your assistants help you lift the bed straight up until it is clear of the fuel tank inlet line. Set the bed to the side, and install the new bed by reversing the process.