Just to clarify, I do not think it is physically possible for a dog sized similarly to Wheaten to jump from the trunk area over the back seats - there is just not enough clearance between the back seats and sloping roof of the car. My Wheaten never was able to do it, although she attempted few times right after I got Model X (the car is now more than 2.5 years old). It is also worth noting that newer cars have larger headrests, and the clearance between the rear seats and roof is significantly reduced as compared to the older cars like mine.
All in all, don't worry about your dog jumping over the back seats from the trunk - she will never be able to do it (unless she learn how to fold down the rear seat back :smile