You can try crate training your pups,
if the crates will fit in your car, or
the doggie seat belt was a good suggestion too if they will take to it.
I trained my large dogs to behave in the car while traveling, but it took 2 people at first.
I had someone else drive, while I sat
in the back of the car.
I gave the dogs the command to sit or
lay down. If they got up, I corrected
them and put them in the sit or down
position. Did not take long for them
to learn. Now while I am driving with them, I just have to give them
basic obedience commands when
we first start up, and they settle down.
(accept for Pepper) hubby lets her all
over the place.