Keep mice & rats out of the engine compartment? (asking for a friend)
You could find where they’re getting in and stuff the area with steel wool, but the best long term solution is outdoor rodent bait stations. You can buy small versions of the ones the professionals use at a hardware store or on Amazon, but be careful where you put them. Raccoons will reach in to try to take the bait, moving the bait stations around, so you might have to look for them if you don’t get the weighted ones, and you don’t want to accidentally poison anyone’s pets.
as @Whitey said- the best way to to give them somewhere else to go- like a trap. the underneath of a car is to open to keep everything out, so set up traps around where you park your car to catch the rodents.
or get a car cat. 
Home and garden centers sell products like this.
We use Messinas rodent stopper in an unheated pole barn where hot rods, classic cars, motorcycles, boats…etc are stored during the winter.
We spray it around the perimeter of the pole barn, and have yet to see any signs of rodents each winter it’s used.
Tester
I’ve heard fox urine is very good for this, but try as I might, I haven’t been able to catch one yet, and then there’s the next step. 
Related, does anybody have suggestions for eliminating fox urine odor? 
CSA
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Does it smell bad? using bounce at the cabins for over winter under doors and in cabinets etc., but might give this a try if it does not smell too bad.
Didn’t you read the product description?
“dries clear and odor free”
Tester
Cayenne pepper sprinkled around under the hood seems to work for me.
RTFM fail, see it can repeat at 30 days, how often are you doing applications?
I store a truck for much of the year in a barn. I place a box of moth balls under the hood where it cannot fall out if I do use the truck.
I first poke a few holes through the box and the odor emanating will keep rodents out.
We transplanted a bunch of Lily bulbs one spring and the squirrels were digging them up like mad.
I threw a couple dozen moth balls in that flower bed and they didn’t go near it again.
Yosemite