She looks really sweet but please if you are going to clip her to a seatbelt, you need a proper harness not just a normal walking harness - that would not keep her safe in the event of an accident
The Gift shop [supporting Rupert's Fund, the Cavalier Collection scheme - both for research into Cavalier health - and Irish Cavalier Rescue] has these wonderful car harnesses which we use
http://cavalier-gift-shop.org/cavali.../prod_216.html
They are excellent as walking harnesses too so you don't need to take them on and off when going out in the car. The panel is neoprene so dries very quickly. It looks firm when it arrives but quickly moulds to the shpe of the dog!
It is comfy to wear too!
It has clips on both straps so you don't have to pull it over their head [which many Cavaliers hate
] Mine have learnt to hold their paw up to have it put on!]
You can also get the camo colours - this is the pink [of course!] - but there is also a blue camo
Something special for your Cavalier and supporting these wonderful causes too!
More on this thread
http://www.cavaliertalk.com/forums/showthread.php?40749-Harnesses-car-harnesses&highlight=ezydog
The Highway Code - Rule 57 says:
"When in a vehicle make sure dogs or other animals are suitably restrained so they cannot distract you while you are driving or injure you, or themselves, if you stop quickly. A seat belt harness, pet carrier, dog cage or dog guard are ways of restraining animals in cars."
If your dog is not happy in a car harness, as Rod says, the next best thing is a sold sided crate, like the airline crates
http://www.zooplus.co.uk/shop/dogs/dog_cages_carriers/crates
Make sure these are safely secured though, either with the use of a seatbelt or using the bungee cords that are available at many shops or motor accessory stores. Most cars have points to secure these to.
One of the books I was reading recently [whose title currently escapes me - senior moment!] has photos of dogs loose in the car, riding on the driver's lap, head out of the window etc,
it then shows photos of young children in the same positions - an amazingly effective way of getting the point across.