Another Alternative
In addition to the methods noted earlier, you could also use “smell” as a way of scaring these squirrels off. Using pet hair in terms of sprinkling it in your car or using a bag of cat litter will give off the scent or expression of a “scarier animal.”
Using Intimidation
The smell of another animal will create some sense of intimidation for the squirrels. However, take note that whatever you put in your car, you do not damage it or break it. For help, ask your mechanic for assistance.