Our two immediately go on their bed which lives between the seats on the floor. We do not have any more than driver/passenger seat belts and I am unconvinced any clip in harness would not injure an animal. tge regulations for restraining children are very tight compared to adults and the size of our dogs would put them in toddler category.
My opinion is they are safer where they are and until a court appearance forces me to do otherwise that is how they will stay. For me this comes under the heading of common sense. Not sure I have the same common sense when I stuff the metal grill pan under the duvet or the TV set though ha ha