We believe a squirrel or mouse did the damage, although it does look like a rather clean cut. Upon very close inspection, you can see what look like bite marks.
Here in Sandia Heights, near Albuquerque, New Mexico, I've had a long continuing problem with pack rats getting up under the hoods of cars parked in the driveway. Sometimes they bring along nesting materials, including fresh cut cactus parts deployed as defensive works. They have also chewed sound blanket material or thermal insulation, and they have also chewed wires. Sometimes it seemed casual gnawing for taste, but in the case of my daughter's Saturn, they twice chewed right through a spark plug wire, apparently in an effort to clear out a bit more living space.
Just once a squirrel took up residence in our garage, cheeping away and chewing up the styrofoam packing forms in an old large telescope box, apparently for nesting materials. When it left after a week (possibly chased away by moth crystals), I noticed it had chewed on the 24V control wire for our central vacuum, severing the conductor.
Here is a picture under the hood of the Saturn after the first spark plug wire incident, showing defensive cactus and completely disconnected wire.
And, the same day, here is some wiring on the other side of the Saturn which just seems to have been gnawed at for taste.
