a seat at the back of a vehicle （车辆的）后座
Idioms
take a back seat
to allow somebody else to play a more active and important role in a particular situation than you do 允许他人领先；甘愿居于人下；退居幕后 Many managers take a back seat and leave recruitment to specialists. 许多经理退居二线，把招聘工作留给专家。