I have a harness for my dog that has a male seatbelt buckle end on it, the other end clips to his collar - designed to make sure he doesn't jump out of my roadster. You can get them at most larger pet stores.

The good thing about it is that if you "accidentally" leave it in on your side and anyone asks, you can always say that you forgot to put it away after taking your dog for a ride, and everyone loves you because it shows that you are a responsible pet owner.