Slight update, but no good news... I am confident I found the wires going up to and back down from the upper marker lights. (Pics to follow later) two green wires and two grounds come from the front of the trailer back to the cavity behind the load center, turn and go through the floor, where they split. One pair continues rearward (presumably to the rear taillights) and one pair turns and goes back up the same hole in the floor, up behind the fridge, through the outside fridge access compartment (with a whole bundle of wires), and into a roof chase, presumably to power the upper markers.
bad news: I can't access that Chase from inside the camper, I can't get to it from outside the camper without scraping the Dicor sealant off the fridge vent grate, and the break in the ground white is somewhere above the fridge access panel. :(
is a *slight* possibility that I *might* be able to fish a second ground from the rear left taillight to the rear upper marker light. There is a cavity behind thetub wall that houses the power cord inlet and the storage tube for the stinky slinky. But it's more than I want to undertake on a Sunday afternoon with rain on the way.
ll try and get the pics up later this evening. I think I want to watch our next camper being built start to finish just to avoid these hunts. :)