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.
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)