We've had very costly damage caused by mice (or maybe voles - they didn't confess). They liked to chew off the wires right at the O2 sensors, leaving them too short to splice. More recently a wiring harness on our Prius was chewed through and cost a tow and a couple of hundred dollars to fix.
We now buy fox urine crystals online, hang a small bag under the hood, and have had no further problems. If you want to keep small rodents away, that works. We bought from PeeMart.com (not a joke). They have urine from an large variety of creatures. I don't have any connection with that site -- in fact, I was amazed to find it.