noun
rumble seat
A folding seat at the rear of an old-fashioned automobile, often exposed to the elements. This is a specific historical term; the word is not used for modern car seats.
クラシックカーの予備座席に座ると、風を感じられて気持ちがいい。
Sitting in the rumble seat of a classic car feels nice because you can feel the wind.
補助席 is a general term for an auxiliary or extra seat, such as a folding chair in a meeting room or a jump seat in a vehicle, while 予備座席 specifically refers to a rumble seat in vintage automobiles.
Compound of 予備 (yobi, 'reserve, spare') and 座席 (zaseki, 'seat'). The term is a direct translation of the English 'rumble seat' and is used in the context of classic cars.