If you've got an hour to waste,
this video has lots of up close
stills of the vehicle, and no mention of the "driver-shooter" theory, so it's not biased.
It looks clear to me that the reflection on the top of the passenger's head that people keep mentioning is actually the rear door handle/arm rest.
That's why it's stationary when the passenger's head keeps moving, and it's also roughly the shape of a gun.