Jax loves his
Amazon.com: Kyjen Outward Hound Car Booster Seat,... . As soon as I got it out of the box he hopped up into it. Like someone else said, he likes having an "assigned seat" and knowing where "his" spot is. He wears a harness, and I clip the seatbelt onto it, very quick and easy. It boosts him up so he can see out the window, but I put a bed in it and mostly he sleeps.
All I have to do is put him in the car and he hops straight up into it. No training necessary.