Dave, my guess would be a hole behind or next to the mirror then in between the roof panels to the pillar and down the pillar like marker lights. And you are correct in saying would take some time fishing it through.
Still a neat accessory to have in a custom camper. I'm cheap and lazy so I would just have my wife hold the dollar store flashlight.
Side note: my parents Buick had one. Station wagon. Can't say if it was approved accessory or if it was something my dad added from J. C. Whitney.