I live in the woods, I have deer mice and bigger wood rats all around.
My neighbor mixes a water-soluble hot sauce with more water and sprays it under his hood, he says it works but be careful where you put your fingers after working in there.
I've heard that nearly any strong-smelling odor will deter rodents. I have been using Lavender oil on cotton balls in my motorhome, no more mice---so far. I bought some cheap Lavender bathroom spray to use in my garage and under hoods, it seems to be working---so far.
If I park a car and know it will sit for a while, I leave the hood open, that also seems to work, we have hawks and ravens who will take rodents home for lunch.
Outside cats definitely work but on cold mornings you must look under your hood before you start it up....
1/2" hardware cloth in air box entrances will work.
But here's the big thing: all animals have "Neo-phobia". Rubber snakes, owls, etc. do work---for a while. Fake predators that move are better, but they get ignored too after a while.
I know zero about sonic devices repelling rodents.
If you use poison anywhere near a car, you may later smell the results IN the car---you do know how I know that, right? I hates those meeces to peeces!