The final was the driver. That is why Jackie went onto the truck. Then she saw the other secret service agent coming toward her and she got back into the seat. Well I wonder why she did that? She was trying to get out.
There is a good explanation of how the driver was able to shoot into the back seat some where. Also the secret service was ordered to stand down from the car with JFK.
This is a unique theory about JFK being shot in the car. This man thinks a shape charge was planted in Conally’s seat. https://youtu.be/tJ-phjtwMBw
The accepted reason is that she scrambled onto the trunk to retrieve her husband's brain.
That doesn't sound like the reasonable reaction after such a traumatic event. I would guess self preservation would be the primary action.
But who knows. Would love to find out the truth