Do not use a rainy drive as an exposure exercise. Wipers add movement across the windshield, a rhythmic sound and sometimes a change in road noise. Keep your dog properly restrained and never distract the driver to calm or reward them. If a necessary journey is not safe for the dog or driver, ask whether it can wait or another plan is possible.
When the car is parked safely with the engine off and another adult can handle the dog, see whether the dog is comfortable with the wipers still. A single low-intensity movement can come later only if they remain relaxed. Stop if they tremble, hide or struggle, then seek qualified reward-based help. The car-sickness article explains what to record if drooling or vomiting accompanies the fear, because nausea and wiper fear may need different plans.
Ask your vet about severe fear or a sudden change in car behavior. Do not turn wipers off while driving in rain to help the dog. Visibility and road safety take priority, and a frightened dog still needs a secure restraint.
Today, avoid nonessential rainy travel and note exactly when the reaction starts. Gentle practice belongs in a parked car on another day, not on the road.
Sources: Dogs Trust on gradual car-travel practice, VCA on car-travel anxiety and safety, VCA on gradual work with a feared sight or sound.