We have a VW Beetle and a Cane Corso, Zita, who likes to ride in the car. When the back seat is folded down, there is access to the two clips that hold the seat up. I took some chain, quick clips, and lots of swivels and made a crosstie with a piece that attaches to Zita's harness. She has enough slack in the tie to sit, stand, turn around, and lie down.
Once she has tried to chew her harness, and then learned not to, she will get the type seen in the pictures above. So far she hasn't thought to chew it, but our Vizsla could chew through anything in minutes, so I am waiting with a cheap harness.
My experience with the dog fences inside the car is that sudden braking will bring them, and the dog, into the front seat.