Sprinkle diatomaceous earth powder in areas of your car inhabited by stinkbugs. Diatomaceous earth is harmless to humans but toxic to stinkbugs. Stinkbugs that come in contact with this powder experience dehydration and die.
Apply essential oils of eucalyptus, cedar or clove in the areas of your car where you observe stinkbugs. Stinkbugs dislike the woody odors of eucalyptus and cedar.
Place floral potpourris that contain chrysanthemums, marigolds and lavender on the dashboard in your car. Stinkbugs are repelled by the powerful fragrance of these flowers.
Examine the doors and windows of your car for any gaps or crevices through which stinkbugs can enter. Use caulk to seal any gaps you find. This will prevent stinkbugs from entering your car.