It just faded away...
First when i got the car all that would happen was a casual face in the back seat (rear view mirror), A outline of someone looking back at you from the back windows if you were behind the car, pulling of the hair (my sister, the girl i was dateing, myself and my mom both experienced someone from the back seat pull there hair), voices like someone is whispering in your ear, and sometimes rarely something will be thrown in the car.
It just slowly went away... First the hair pulling stopped, then things getting thrown around, then the outline of someone, then the rear view mirror... Sometimes you will hear whispering but not often 8) .
I named the entity Freddy
