From the factory the bed was grounded with mounting bolts. They have had 50 years to rust up. You can cure it by running new ground straps where there are none.
I don't know how the '61 is wired but on a '54 the battery is grounded to the frame. There are two ground straps from the frame to the engine. Another runs from engine to cab. A lot of trucks will have had some of the straps left off after all these years. Make sure yours has straps in all these places and clean the connections. Then add one at the bed.
If the sockets don't ground to the bed you can remove them and clean the mating surfaces.
