Yes, mothballs help, especially with bigger rodents, like squirrels and chipmunks, which I would bet are what cached the acorns in your airbox.
I've also heard that mice will use mothballs in their nests, which is discouraging.
I've learned to not cover things, because the cover is just what the mice and chipmunks are looking to get under. Unfortunately, cars provide enough cover and hiding places that are irresistable to 'em, so moth balls are the next best thing. And traps, if you remember to check 'em periodically.