You could always put a blanket or something down so they don't mess up your seats, or you could put them in a kennel when they're in the car. I know it sounds mean, but they'll be fine
I would not talk to the person directly, because that may just cause problems. I had the same thing happen to me, I talked to them, nothing changed. Then, they knew that it was me that called the cops. It was annoying.
I have to drive all the way to Idaho from Washington. I know I'm lucky though, because many people have to drive much further or even fly. The drive is nice too, so I can't complain