My own experience
I've experienced this phenomenon once in my life and thought it interesting, but never gave it much consideration until I read about Shackletons "third man" experience and the factor as a whole. My wife and I were driving from Los Angeles to North Pole, Alaska, a trip that we did in just over 72 hours, as we were driving virtually non-stop, other than to eat or go to the bathroom. She drove from LA to the Canadian border, and I drove from the Canadian border in Washingon state to our final destination. As you can imagine, we were very tired, and to make matters worse we passed through a very hilly area in Canada during a snowstorm, at night no less. My wife was curled up in the passengers seat sleeping, while I fought to keep my eyes open. If you've driven at night during a snowfall you realize the hypnotizing effect it can have on you, particularly when your headlights are illuminating the hundreds of pieces of falling snow whizzing at the windshield. Coupled with the seemingly endless twists and turns of the mountain passes and heavy eyelids, it seemed as though we were a bend away from getting stranded or wrecking our car in the middle of nowhere.
I don't recall when I first noticed the woman giving me directions, but as I drove on her voice calmly gave me minute by minute instructions- "slow down around this turn", "now back the other way, don't overcorrect", and the like. She was a middle-aged Mexican woman, heavset, with her hair pinned up neatly in a bun and wearing a cleaning lady's uniform- white apron, paisley blue dress with a collar, and flats. I didn't so much see her in front of me as much as I was aware of her in the upper part of my forehead, as though she was hovering just over my line of vision and projecting herself onto my frontal lobe. I was unable to foresee the lay of the road and adjust accordingly like most drivers do every day in similar situations. I was responding only to her commands as she gave them, and seemed only able to do so as she spoke them to me. After an indeterminate amount of time, we passed through the storm and on to clearer roads as day broke. I recalled my experience to my wife, who had been asleep throughout the night, and we both laughed it off as a sleep and nicotine deprived delusion.
What struck me most during my experience was the serenity that the lady exuded; her commands, as I mentioned earlier, were more akin to gentle admonitions like a grandmother coaxing a small child. "You ought not do this" or "You ought to do that, my dear". I haven't experienced anything like that since; I'm not sure if she were a guardian angel or simply a figment of my over worked psyche, but nonetheless the Mexican cleaning lady got me through the worst driving conditions of our trip unscathed.