back-seat driver

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A passenger who constantly advises, corrects, or nags the driver of a motor vehicle.
• noun A person who persists in giving unsolicited advice.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person sitting in the back-seat of a car issuing (unsolicited and usually unwanted) instructions to the driver.
• noun figuratively An opinionated person who offers (unsolicited and often unwanted) advice on the best way to handle a situation.