The king of the car is the driver. The driver is such because the driver has all the power. Riders may complain, cajole, or threaten but unless they are driving they cannot determine where the car goes, the speed at which it goes or the route it takes to get there. This is a simple rule of might makes right, and the driver has all the might.
However, the driver cannot easily control the car and the media, so in fact riders do have some power with which to fight the driver, passively. This power, like so many tyrants have found to their demise, is the media, which in the car is represented by the radio. It is a tacit understanding that the front passenger has radio (media) control, and although it may not be as useful as a printing press in eliciting a rebellion it can be used to annoy the hell out of the driver. To a lesser extant the AC and vents can also be used to make life for the road tyrant an uncomfortable throne to sit upon.
Then there are the outlying provinces. Like many colonies, the less desirable of the nation are sent to the borderlands. (for less desirable please read children and/or your spouses crazy friends.) There they often are left to their own devices and entertainments, blissfully doing whatever it is they do in the back seat until they require a higher power to lawfully intervene. Here the passenger once more comes into play as police force. Should they desire to thwart the tyrannical driver's regime they may elect to instead support the insurrection and chaos of the back seat provinces requiring the driver to avoid annoyance and discomfort by acquiescing to the wishes of the passenger.
The balance however is delicate and the passenger themself must be careful to avoid distracting the tyrant too much from the rule of state. This may result in the very least as an unforseen delay when the car stops for some "alone time" and in the very worse the crushing of the nation under the 18 wheels of absolute dictators.
So you see, the auto is a unique microcosm of human government that illustrates that at its most basic form any government be it democracy, monarchy or tyranny or other wise must always maintain a balance or respect for the people governed and those that govern. When this balance is upset a state may find itself simply road kill on the interstate of history.