.. when the tailgate is all the way open. Not a linkage binding issue, as it wipes strongly when the tailgate is open. All the other tailgate functions work flawlessly whether tailgate is open or closed (KOW). I made a visual inspection of the harness on the wiper side and see no cracks or breakages in the wiring, though the plastic sheath itself has broken. From what I can see of the ground wire it appears to be ok as well.

I suspect that there is a break or weak spot in either the wires powering the motor or the ground strap that I can't see. But wouldn't having the tailgate open "stretch" the wires further apart, thereby making it more likely to not work while the tailgate is open?