I put a harness on my dogs in the car, and then have them attached to a seatbelt with a tether. They cannot physically move into the front seat. And when I open the car door, they cannot jump out either. I attach the leash, unclip the harness, then release them to jump out (actually, Rosa gets picked up and set down since she can't do the jumping with her bad hips).
Maybe until your pup gets some more self control (that comes with age and training), a tether for car rides might be a good idea.
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old