One who sits on the furthest row of seats in a vehicle like a bus, a van, or a car. The term implies (often ironically) that those who sit at the back are "cool" and/or "mischievous".
Those backseat bandits are causing mayhem again. What is about the back of a bus that gives kids such a powertrip?
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.