Squirrels chew on car and truck wiring for a variety of reasons. The first is a theory that the wires taste like saline when chewed upon. Perhaps the better reason is that squirrels must constantly chew to keep their teeth worn down to manageable levels. Rodents teeth continue to grow throughout their lifetime.
Squirrels, rats and mice will chew on the plastic of windshield wiper and coolant overflow jars. Radiator shrouds and air filter boxes are another common item chewed on. Brake fluid reservoirs as well as rubber hoses and belts are damaged too.
There is not a lot you can do to protect your car or truck in advance of an attack on the wiring as rodents are going to climb up and do the damage regardless of your best prevention techniques.
If squirrels or other rodents are chewing your wires, visit A All Animal Control for a location nearest you for assistance.