[LunarBerry]

Well there are pheromone sprays that has calming effects that you can spray on the booster seat, etc. they have lavender scents so it's not too off putting. Thundershirts might also work (aside from thunder times). Or trying to make being in the car pleasant. Like going to really awesome places (like a park or something) or sitting there and just hold them while someone drives. My yorkie girl has no issues with the car just as long as it's not crazy bouncy (I have held her all the way home when I picked her up and have always tried to take her to fun places and that worked for me) and my other dogs are obsessed with getting in the car lol