It’s gotta be tail between the legs… hole in the seat would be cumbersome to feed your tail through, a slot to one side would compromise crash safety. Up sounds like you’d slide around in the car seat when cornering.
I feel that tail holes in chairs in general would simply be commonplace if we had tails.
It reminds me of me thinking the same about people with dreds when they go to bed.
tail hole on the seat
I think it depends on the animal.
I'm betting @mmmatch.es has a very specific and opinionated answer on this.
I put mine through the hole in the seat.
wouldn't "tail on the side" make more sense on the other side, where there's a bit more room between the seat and the door?
Imagine that who designed the car seats also has a tail and voilá.
I like option 1 and 4