車裡的那個位子
這是畫面本身，其他意思都從這裡借出去。
noun · A2
The seat, or row of seats, behind the driver in a car, taxi or similar vehicle. The people there travel with the car but do not control it.
後座
核心畫面
Picture where you are when you are in the back seat of a car. You are inside, you are going exactly where the car goes, and you will arrive when everyone else arrives — but the wheel is out of your reach. That one picture holds two very different people. One of them accepts it quietly and lets the driver drive. The other leans forward and keeps calling out the turns. Every meaning of back seat is one of those two people.
可以想成你此刻就坐在車子的後座。你在車上，車去哪你就去哪，時間到了一起下車。只是方向盤碰不到。這個畫面裡其實坐著兩種…更多收起
可以想成你此刻就坐在車子的後座。你在車上，車去哪你就去哪，時間到了一起下車。只是方向盤碰不到。這個畫面裡其實坐著兩種人。一種安安靜靜讓開車的人開，那就是 take a back seat——退到次要位置。另一種身體往前傾，一路喊著該轉彎了、該煞車了，那就是 back-seat driver——手上沒有方向盤，嘴巴卻沒停。back seat 的每一個意思，都是這兩種人其中之一。From the word’s history
The source records the compound simply as back + seat: the seat at the back. Everything else is that literal place used as a picture.
來源只記了一句：back（後面）＋ seat（位子）。所以它原本就只是「後面的位子」，其他意思都是拿這個位置當比喻長出來的，沒有另外的來歷。
Wiktionary entry for “back seat”, etymology_text “From back + seat.” (packet source_ref kaikki:3f6f69a190036ae4:1159967941:12470)
Three branches
自己退到後面
同一個畫面，但你是那個安靜的乘客：主動把駕駛座讓出來，人還在車上，只是不再決定方向。
noun · B2
To accept a smaller, less important part and let someone or something else lead. A thing can do this too: a plan or an activity takes a back seat when other things become more important.
退居次要位置
坐在後面卻想開車
同一個畫面裡的另一種乘客：一樣沒有方向盤，卻用嘴巴在開車。這兩個意思都只活在 back-seat driver 這個完整說法裡，單獨的 back seat 沒有這個意思。
noun · B2
A passenger who keeps telling the driver what to do — when to turn, how fast to go, which lane to use — when the driver has not asked for any of it.
愛指揮駕駛的乘客