Greg,

Your assumption is correct on the grounding. The brass lamp housing is swaged into the assembly. This should be a perfect ground from housing to taillight assembly. The taillight assembly is then bolted to the bed. This connection must be clean and secure because it is part of the ground path too.

The bed must be electrically bonded to the frame which is/should be bonded eventually to BATT negative...viola, the path is complete.

The lead buttons on the bottom of the light bulb must be clean..you can cut a little piece off of each pimple to get them shiny and fresh.

A ohmmeter should verify all these connections pretty easily.

Stuart