Use a proper dog crate on the back seat secured by the seat belts.
Good crates use a plastic tray on the floor and we use "Vet Bed" to line it. Very absorbent and easy to wash. In the early days, take a spare piece.
Also take a towel or two and plenty of drinking water (for the dog) if it's a longer journey.
If you start from early in the dogs development it will soon get used to it but try to make the first few journeys relaxing and not too long initially.
Oh and don't forget the poo bags.
