My ds needed to hold someone's hand at that age, would scream the place down if he was in the car seat on his own in the back of the car, but was much calmer if I sat next to him and held his hand.
Once he was older, it flipped and he went from hating the car seat, to loving it, was the only place he would fall asleep for a while, if I remember correctly, I think the change was around 6 months, but it was years ago.
Does your ds settle in his pram? Could you look at renting a lie flat car seat for the trip to see if they prefer that?
Do they have anything like white noise/rain sounds etc at home? Could you play something similar in that car if they do.
Are you feeding them in the hope they will sleep for the journey, but not allowing enough time for the feed to settle so they are going in the seat windy?
Hope you work it out and finds something that helps them settle, a grizzly baby is no fun, but also very common, so don't worry, it will pass!