Had a mouse/rat problem around the house (there was a new housing development going in right behind our house). So all the construction destroyed their homes.....they would come over the block wall at night and eat anything in my garden or open trash!

Someone told me to buy the old rat/mice trap (wood with spring) and put "slim jim'* pieces" where the cheese should go.

Killed a rat or more a day until problem was gone.....I am sure anything like a smoke sausage would work. Slim Jim was hard to pull off tho and triggered the spring action.

So if soap does not work.....try setting traps around car. You will have to clean and reset traps every day, but Slim Jim'* are cheap!

Some cats just aren't mousers. With a good mouser around, the only mouse problem you will have is what to do with the half eaten dead ones.

If you set traps around the car, mice also love canned dog food. The stinkier the better

Remember, on mothballs, you only need a couple, or even just use one. It'* not that bad.

I thought about traps, but I don't want to disturb the birds. They eat bugs. And then there is the wide variety of other wildlife hanging around like raccoons, squirrels, turtles, and basically every other sort of beast that is indigenous to this area. It would be my luck that I would get in trouble for maiming a raccoon with a mouse trap. That or it would just make them mad and they would poop where I would step in it. I am all for leaving the animals alone if they will leave me alone, but when they attack my cars they have to die or get run off.
So far the only problem seems to be rodents small enough to fit under the breather and around the carburetor of my Lemans or under the plastic cover on top of the engine on my Bonneville, which leads me to believe it is mice.
I used to have a metal trash can outside that the raccoons liked to knock over in the middle of the night. It sounded like two mules fornicating in a metal barn. They seemed to always know to wait until I was asleep before they knocked it over.

The mothballs are actually toxic. I have been hearing about this from a variety of different sources. My health is bad enough already, I don't want to risk getting worse.
mothball warning
I'll try some smelly soap and see how that works. I think the owls would rather have them alive.

Some people are highly allergic to mothballs. My mom is, for example. And I certainly wouldn't want the smell of them in my car.

I like the idea of the irish spring soap.

And I was immediately going to say "Get a cat" but I see everyone else beat me to it. And you have cats. And they don't work. Your cats are obviously not mousers. You need a mouser.

Personally, I have dogs, but I don't think they have anything to do with keeping mice away. In fact, I think mice would make my dogs nervous, and they'd come to me looking for protection.

Someone must know an Orkin guy. They have lots of different things for rodent control. I think they have some things that are little box tunnels, that birds wouldn't go to. They put poison bait in those for the mice. Maybe I'll ask my Orkin guy when he comes around next time. (We have an Orkin guy for ants, which are bad in California.)

Take the engine cover off of your engine. Doesn't really do anything and take it will take away a "hiding" place for the critters.

I didn't say I have cats, but there are cats around here all the time.
The soap seems to be working on the old cars. I didn't put any in the Bonneville because I have been driving it daily. I assumed they would stay out of it. Wrong.
For some reason I decided to check under the hood of my Bonneville before I drove it today and I found a nest in the same spot made of under hood insulation.
So now the question is, how do I attach the soap? I would rather not have to take it out every time I drive my car and then put it back when I get home. In my old cars, I have the bar of soap resting on the AC box at the back of the engine compartment. The Bonneville doesn't have any where to put it that it won't either fall off or get too hot.

What are the instructions for properly mounting the soap in a Bonneville?

I propped up the soap next to the battery on the Bonneville, using the battery cables to wedge it in. My two old cars have a spot on top of the heater box that I set the soap on top of, and when the hood is closed the soap bar is held in place by the electrical plug on top of the heater box. This seems to be working to discourage the nesting, but they are still getting in there and pooping and leaving nut shells. It is as if they are considering nesting but the smell of the soap runs them off.
So I'm bringing out the big guns, I got some d-CON. No more mister nice guy, these rodents are going to die. The intake on my Lemans is covered, I put Tom Cat nibble blocks & d-CON bait pellets all over it. There is enough poison on it alone to kill a dozen rats. They're doomed. I just hope they don't crawl inside it to die. The poop smell is bad enough.

steel wool.

Right! Steel wool with the wet soap rubbed into it -- nice and gooey and smelly. Or shake a few bits of soap up in a "Fantastic"-type sprayer full of water. Then keep spraying it all over (avoiding rustable parts, of course)

Last resort, since you don't want to use traps and if the poison doesn't work, use small humane traps with stinky canned dog food in it. Mice love it. You just have to shake out the traps in the park once in a while (or take vengence and dump 'em off near the car of the neighborhood "pest" -- there'* always one .)

But if you're finding fairly large nutshells, it could be chipmonks (chipmunks?).