If the container of gasoline were, say, a plastic 1gal. milg jug, it would have melted along with the other plastic in the car. Maybe it would not very be very easily recognizable without lab tests of the melted items?
If she had gasoline on her...would a container under the seat have caused the gasoline to splash/explode/spray from the back seat, or from under the front seat?
Aren't there certain chemicals which, when mixed with gasoline, cause immediate combustion? Like Chlorine? If that is the case, is it probable that such a rigged container could be thrown through an open or blown out window into the car, even at highway speed?
Here is the scenario I envision...
Another vehicle pulls very close alongside her while no cars are nearby to witness the event. They blow out the side window (probably with something other than a gun...hammer, tire iron, whatever, so that a muzzle flash cannot be seen by opther drivers on the road) and throw a burnable container mixed with gasoline and another chemical into the car. When it hits the front seat, or possibly her, the chemicals react and the whole car goes up in flames. This might explain how the gasoline got on her clothes and how she was burnt so badly. The perps accelerate away, as she begins slowing down in panic, trying to douse the flames and control the car. As she is slowing down, other drivers begin passing her car in flames and are so enraptured by the sight, they never notice the perps up ahead. At night, all the perps would need is a gap between cars on the road of about a quarter mile and the following cars wouldn't see the events clearly as they happened and the oncoming cars wouldn't be able to see either, as their speed and perspective would be a hinderance.
just speculation here though, as many facts are missing...
Hopefully they'll put out radio ads for anyone who may have been travelling that road that night and what, or who they may have seen.
The other avenue of speculation is "was she unconscious and dowsed with gasoline"? If the fireball is quick enough, this does not need to be the case. I talked briefly with a friend of mine who says that she's been told that she would've died in her car fire if someone hadn't pulled her out (as it was she got considerable scarring on her hands and arms that she still has 20 years later).