Hi Michelle!
I am not surprised that Kramer reacted that way in the car since he hasn't shared a car with another dog before. Maybe he had space issues you weren't aware of before since there was no opportunity for it. Or maybe he was sleeping and she startled him. At playgroup, he was just running around with the other dogs, not laying down with them. When we are on long car rides with Breeze and Beka, Beka gets snarky sometimes if Breeze is changing position and steps on her, and as you know, they've been together for three years. I think you just need to give Kramer time to adjust and do the same back seat/back of the car arrangement until they get used to each other. He is such a happy boy, he will probably be just fine once he gets used to the idea.
See you in July!
Kathy