The phrase “to take a back seat” means: To take a seat or occupy a position among those of little importance; to practice humility or become humble; to go ‘way back and sit down.
There’s little doubt that this expression was an outcome of the natural characteristics of Americans.
The aggressive, those determined to be heard on any subject up for discussion, inevitably plant themselves toward the front of any assembly, so that they may rise and be seen readily by those whom they wish to persuade.
The humble, those who instinctively avoid the limelight, as well as those who hide their lights under a bushel, willingly keep in the background.
A back-seat driver, however, is a horse of another color. He, or usually she, knows not the meaning of the word “humility.”
On the contrary, she is the aggressive one, dictating from the rear seat what the actual driver in the front seat, usually her husband, should do, what road he should take, how slow he should drive, what he should do in any emergency, and so on.