Ed, after many trips to the Wapakoneta, assembly plant, while at the ramblin'pushers maintenance rally, I learned that "all" wiring is run in the upper right corner of the roof. They had a floor plan of the coach that stood vertically that they were working on and manually ran each wire from the rear corner to the location of the device. I don't remember if the wire is run over or under the insulation but it should be easy to find it by removing a ceiling light. The bad thing is replacing a failing wire, though the talk of using the ductwork as a path seems convenient.
Gary 05 AMB DST