The term backseat driver denotes a passenger in the rear seat of a car who gives the driver unwanted advice, hence, figuratively, a person who is eager to advise without responsibility.

(This term is similar to armchair critic, attested in the mid-19^th century, denoting a person who offers comment or criticism on a subject of which he or she has little or no practical knowledge or experience.)

The earliest instance of backseat driver that I have found is from an article about the horse races at Rush Park on Saturday 4^th July 1891, published the following day in The Sioux City Journal (Sioux City, Iowa, USA)—the term is apparently used figuratively, and its precise acceptation is unclear:

If at all interested in human nature, the patronizers of a race course never fail to receive the full value of their entrance money, for there is no richer gold mill to explore than is found within these gates. All sorts and conditions of men—characters and characters, until you begin to think that every man has a romantic history. Their past, present and future somehow or other leaks out, and between heats these choice morsels are passed around for the refreshment of the crowd.
[…]
“Who is that awful looking man in a union suit of blue jeans, and is he in a glass cage?” “That is W. H. H. Colby, of Fort Dodge. His glasses are almost as large as a base ball catchers’ muzzle and made after his special order, though you won’t find another man on the turf that would care to pattern after them. However, he is no back seat driver and knows how to make a few hundred thousands.”

The second-earliest instances that I have found are from The Anaconda Standard (Anaconda, Montana, USA) of Sunday 15^th August 1915, which gave “a definition of a backseat driver”—this text also contains an early instance of the noun backseat driving:

The sex is generally feminine, and the inspiration is a combination of fear and hope. The backseat driver takes it upon herself to do all the duties of a chauffeur except, of course, run the car, which is a minor matter.
She endeavors to push her French heels through the floor every time she thinks the car should be stopped in the imaginary motion of applying the brakes. But the most confusing habit of those chronically addicted to backseat driving is signaling with the hands as if to stop or turn a corner or change the course in any direction.
The greatest trouble is that there are usually two or three backseat drivers per car. Approaching from the rear one of these cars loaded with the average family of backseat drivers is as confusing as trying to select the prettiest girl in the Ziegfield [sic] Follies. The occupants of the back seat have the indecision of a chameleon placed on plaid and hands usually stick out in every direction, indicating it may change its mind and course and even back up at any minute. Judging by the semaphores in the back seat the automobile is as uncertain in its intentions as the millionaire’s daughter who is trying to make up her mind whether to elope with the chauffeur or a social gangster.

The earliest figurative use of backseat driver that I have found is from the Jackson Daily News (Jackson, Mississippi, USA) of Monday 12^th December 1921:

Back Seat Drivers

A back-seat driver is the pest who sits on the rear cushions of a motor car and tells the driver what to do. He issues a lot of instructions, gives a lot of advice, offers no end of criticism. And doesn’t do a bit of work.
You find back-seat drivers in other places than autos. They are carping continually, in domestic, business and public life, telling how the thing should be done and what they’d do if they were running things.
We have a lot of back-seat drivers right here in Jackson, and elsewhere in the commonwealth.
They are the class of men who cuss the courts for unfair judgements, yet they dodge jury service whenever possible. They criticisze [sic] public officials who neglect their duty, yet they fail to take proper interest in the poltiical [sic] contestss [sic] that put such men in office. They assert that civic organizationss [sic] are doing nothing whatever for the growth and up-building of the community, yet they neglect to attend the meetings of these bodies and do their share of the work in promoting the material welfare.
In nearly every respect, the back seat driver is a darned nuisance. He doesn’t even possess the merit of being a good critic.

The following review of The Back Seat Driver (1928), by the Australian novelist Alice Grant Rosman (1882-1961), is from the Sheffield Daily Telegraph (Sheffield, Yorkshire, England) of Thursday 29^th March 1928:

Interference.

Nearly everyone is acquainted with someone or other who persists in interfering in the affairs of his or her neighbours, and friends, and giving unwanted advice. In “The Back Seat Driver,” by Alice Grant Rosman (Mills and Boon, 7s. 6d. net), Constance Marchmore, a spoilt beauty, believes herself capable of controlling the destinies of other people, including that of Bill Trevor, the handsome young fellow who is engaged to marry her. In the end she is left with the knowledge that in trying to manage other people’s destinies, she has mismanaged her own. It is a cleverly-contrived story.