A puppia or any harness designed just for walking is not actually safe as a car harness -- remember that a dog can be thrown forward on an impact of only 30mph with a force up to 20 times its weight in the first foot it would travel after impact. For an adult cavalier weighting 15-20 lbs, a Puppia would be unlikely with its construction to restrain a dog thrown with the force of 200-400lbs against that harness and its ring. Either the harness would break, or it could potentially hang the dog or decapitate it due to the way the Puppia neck is constructed. If you look at car harnesses they have a completely different construction and would not be able to hang the dog. It is also quite easy for many dogs to pop right out of a Puppia if they were thrown backwards -- Lily can easily escape hers if she simply wants out of the car. :
The only harness I know of so far that is actually safety tested in proper car tests and exceeds human safety standards, is the Ruffrider: www.ruffrider.com . I am sure some others are adequate, too.
For a puppy I'd just use a correctly sized crate til adult size unless you want to buy a succession of harnesses as they must fit properly.