A car usually is equipped with four wheels. And below "wheel" doesn't mean a steering wheel.
(1) a wheel of a car
Does (1) implies there is more than one wheel?
(2) the wheel of a car
Does (2) implies there is only one wheel?
(3) a backseat of a car.
Does (3) implies there is more than one backseat?
(4) the backseat of a car.
Does (4) implies there is only one backseat?
Interestingly, I found stock photos often uses "a X of a Y" like the case below
Businessman sitting on a backseat of the car using smart phone.
Businessman sitting on a backseat of a car using smartphone and looking around.