48loadmasterguy,
Are the lights attached to the roof with screws, nuts and gaskets. I would make sure at least one screw per light was bonded clean and tight with the light assembly and the sheet metal. I would do just as you said and run the ground from one to the other..to the other...to the other, then continue the wire down to anywhere you can access it, and attach it to a good clean ground.
16 or 18 gauge wire would be more than adequate for this task.
Have you scratched around inside the non-working lights to see if you can get them working before you go through all this work. In a perfect world, the fixture should be grounded to the cab through its mounting screws or bolts. Maybe they are oxidized or cruddy and just need some TLC.
Stuart
