the insulation of most newer cars or newer wires are commonly made of soy plastic. they like the smell and gnaw on them. the older plastics which were made from petroleum by products were rarely, if at all, chewed by rodents.
in most parts of europe, it is not only rats that chew on the wires, the ferrets and squirrels as well. the europeans usually hang a small bag of cat hair or something from the cats and place them in fabric or stockings and hung inside the engine compartment to deter the rodents. there are some sprays to discourage the rats from infesting your car, the only thing is if your pets ride in the same car, these pets feel uncomfortable.