Be sure your dog is comfortable with whatever setup you buy.
Years ago my wife and I bought a harness that attached to the seatblet and was called the 'Dog Sitter'. The dog seemed fine with it while driving, but I ran in to a shop for 3 minutes and when I returned to the car he had chewed through the seat belt, wrapped the seatbelt around his legs, and panicked to the point that he relived himself on the front seat.
We re-named the product the 'dog s*itter', and did not use it again.
(the car was an old beater, so no major harm done)