A rat-proof garage.  A pack rat only needs an opening the size of a nickel to enter a garage.  They are excellent climbers and can enter high or low.  A poorly sealed garage is worse than parking outside, since rats are attracted to an area they can enter, but larger predators cannot.  

If a rat-proof garage is not available, the vehicle should be parked in the most open area possible, ideally with at least five to ten feet of totally clear space on all sides.  

If the vehicle must be parked in a carport, there should be no other items stored in the carport including boxes, yard tools, and trash containers.  Any additional cover from stored items makes the area more attractive.   

If the vehicle is not in a rat-proof garage, keep all vegetation in the immediate area well trimmed to eliminate as much cover as possible. Do not store items  that rats can hide behind next to walls.  Lighting is effective as a deterrent only in areas where there is minimal cover.  

For most vehicles, leaving the hood open takes away the sense of enclosure and protection that the rat is seeking.  If you choose to leave the hood open, be sure to disconnect any internal engine compartment lights to avoid a dead battery in the morning. Also, secure the hood with some type of strap if high winds are a possibility.  

Inspect the general area for nearby pack rat nests.  A single nest may produce 10 young a year who need to find new homes (like your car) as they mature.  Removing nearby nests can significantly reduce the risk of attack.  Before removing a nest, the resident rats should be trapped. A homeless rat will seek a nearby alternative shelter—most likely your car!  

Remove all loose material and droppings 

Soak all urine stains with a 50% solution of pine oil (Pin-Sol®) and then use a high-pressure hose to clean everything off. Pay close attention to hard-to-reach areas.  You may need to remove the battery to clean behind and underneath.

Once the vehicle is clean, the hood should still be left open as a further deterrent for at least three weeks to be sure the scent is completely gone.   

Park in the most open area possible away from vegetation, rocks and other objects.

Poison Bait.  Poison bait is rat food.  If you want to attract birds, you put out bird food.  Putting out rat poison attracts rats.  They may eat the poison and die or they may collect it for their food stores. All poisons take 5-10 days to work, so even if the rat dies there is still plenty of time to do damage and “mark” your vehicle as an excellent nesting site.  In most areas, there are too many rats and they breed too quickly to ever be poisoned to oblivion.  In addition, bait boxes (required by law for bait used outdoors!) provide perfect places for rats to hide in and leave more of their scent. Worst of all, as the rats get sick, they are easy targets for predators. The poison in the rat’s system may also kill the very animals that eat the rats, particularly hawks and owls.  See the Problem with Poison page for full details.

Light placed under the car.  A light under the car is minimally effective since the rats quickly go up into the dark engine compartment. It is far more effective to open the hood and hang a light from the hood lighting up the entire engine compartment.

Moth balls.  A small quantity of moth balls is of minimal deterrence.  For a stored vehicle, a large amount (several boxes) may work, but will need to be removed and the car completely aired out before it can be driven.  

Ultrasonic devices.  Rats will nest near, next to and even on top of ultrasonic devices.  

Commercial repellent sprays.  Pack rats ignore them.

Potato flakes.  Pack rats will not actually eat the flakes, swell up and die.

Ivory soap.  The smell does not bother pack rats and if they eat the soap it will not harm them.

Dryer sheets. The smell does not bother pack rats.

Predator urine (bob cat, wolf, cougar, human or otherwise) The smell does not bother pack rats.

Gum.  Pack rats will not eat the gum, choke or die.