This jumble was spoken by someone who was relating that she had no been motivated in the past but was now ready to move forward. It is a mashup of “on the back burner” (postponed or suspended) and “take a back seat” (occupy an inferior position; allow another to be in control). Both idioms contain the word “back”, causing I suspect the confusion. Not sure I have left my motivation on the back seat of the car, but certainly my wallet and sunglasses. A big thanks to Lynn Hannula Johnson for hearing this one and sending it in.