Fear of having an unknown person in your backseat (Doesn't matter where you are sitting, the person could be in the trunk for all you know)
Fear of a Killer in the backseat.
Larry: Have you seen the way Rob has been driving lately?
You: Yeah how he just heard about the story of the killer in the backseat.
Larry : Sounds to me like he has a case of extraserophobia
You: Poor guy, hes always checking in the backseat while driving hahaha