I’ve had good results with mothballs. But that was inside a vehicle (in my case, a trailer) - might not be quite as effective under the hood, where ventilation is likely to be better.
I’ve heard peppermint oil is very effective, though I doubt it will actually drive mice away from existing nests.
When I bought my house it had been sitting empty for over a year and was overrun with mice. After trying all sorts of methods, I found the perfect, and most effective device at my local hardware store. The rat zapper 2000
No more mousies. It kills them, but it’s an instant death.
Apparently if it’s left for long in one location, rodents will start avoiding it, so it would probably work if you left it under the car.
I wonder if one of those gimmicky sonic devices would work? also, if they’re under the hood, they are probably in the trunk and inside the passenger compartment, too. If you aren’t averse to poison, D-Con works well. I’ve seen other non lethal products at my local big box hardware type store, but I can’t recal any names at the moment.
On a side note, when I saw the thread title, I read it as RAPPELLING mice and had a vision of the little creatures descending a wall with ropes and tackle, wearing guns and packs, all the while chanting, “Hut, hut, hut, hut, hut” like the swat team in the Blues Brothers movie.