In almost all instances, rats will be found in the engine bay. Favorite hiding spots for rats in the engine bay include the areas behind the battery, under the engine cover, between the headlamps and the radiator, between the firewall and the engine block. Rats usually enter the engine bay jumping onto the axle or drive shafts and making their way deeper into the engine bay. Although the cabins of most modern cars are well insulated, there are spots like the fresh-air vent of the air conditioning system or the rubber grommets on the firewall, especially around the floor-mounted pedals that can let rats in.
Once inside a car, not only will the rat urinate or defecate everywhere, it also leaves a foul smell in the interior. If by chance a rat dies in the air conditioning system, the only way to get rid of the smell is to professionally get your car cleaned and your whole air conditioning system serviced thoroughly. Rats can also chew on upholstery and carpets ruining the interior look of your car.
Rats usually end up chewing things like hoses, belts, wires, bulb holders, rubber inserts, connecters and fasteners. If god forbid, a rat chewed through a key cables that connects your car to its ECU, the resulting costs could be in the thousands. Chewed though wires can also cause short circuits which could end up being potentially life threatening.
The best home remedies to counter a rat problem are naphthalene balls and raw tobacco. Buy a good amount of the cheapest or worst quality of chewing tobacco one can buy and sprinkle it generously in your engine bay. Before you do that though, thoroughly wash the engine bay at a service centre. You can also generously distribute naphthalene balls in your cars engine bay and boot as they too help keep rats away. Spraying the engine bay with phenyl too helps repel rats. If you park in a basement, make it a habit of completely disinfecting your parking spot once every two weeks to keep rats at bay.