Hi Staci, we use the "Easy Rider" car harnesses when we go on small trips, they are padded in the chest, Daisy weighs about 15 pounds and she has an xsmall, Perry weighs about 28 pounds and he has a small.
We got them at Petsmart. If we go on extended trips we put them in their crate for safety.
The easy riders hook right into the seatbelt, and keeps them from jumping up on the window or up and down, however they can lay down or sit up whichever they prefer.
I never take them in the Explorer without the harnesses on. Good luck, hope you find what you need.
Jackie