Allen,
The item you’re referring to is the sheathing that goes over the taillight harness. You are correct in that the harness enters the trunk from the drivers side. The sheathing covers the harness all the way to the passenger side.
I don’t believe that any vendor carries the sheathing. In fact, when you purchase a reproduction harness the sheathing is absent. I took the sheathing off of my old harness and reinstalled it on the reproduction harness in order to make “technically” correct.
Bob