the euro wagons that had these had factory holes drilled in the inside of the unitbody rails-on the outer sides. I saw one once, ask the forum if someone can make templates-(out of tin foil).
you then would pull the upholstery covering each side rear well and use the templates to drill the holes.

as far as weather the tow bar is sandwitched between the body and bumper brackets--or weather the towbar is installed last over the bumper brackets, I cant tell you but I think its the former.