back-seat driver

noun3

1. •
   A person sitting (typically) in the back seat of a car issuing (unsolicited and usually unwanted) instructions to the driver.
2. •
   (figurative) An opinionated person who offers (unsolicited and often unwanted) advice on the best way to handle a situation.
3. •
   (old-fashioned) An occupant of a large vehicle, such as a firetruck, who helps steer the vehicle from far in the back.

   • When New York pitcher Vernon Gomez retires as a smokeballer he wants to become a smoke eater. Here he gets a tryout as a back-seat driver on a hook and ladder truck at St. Petersburg[…] — 1914