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If the behavior has recently started or she did not used to do this you may want to have her examined by her vet. Dog breeds that do not normally droll and then start tend to have a infection or a problem with their teeth. Though foreign objects and poison can also contribute to the problem. I would be concerned with her teeth if this recently started.
Unfortunately there isn't much you can do to prevent the drooling except to make sure she doesn't eat a large meal for at least 2 hours prior to riding in the car. This will cut down on the problem drastically.
The main cause of the drooling is the air swishing into her mouth from the open window.
As for a shield or something there is no such product on the market (Though it is a great idea for a new product). You could take saran wrap and wrap it from the top of the window to the bottom of the car to help. Otherwise washing it after each trip is your only option .