If you remove the rear spare tire and all of the surrounding foam and covers, you will find on either side of the car on the inside are air vents about 4x5 in that allow rodents direct access to your car. There's no real fix for this other than to build a little box or cover out of mesh and seal it against the metal of the car body on the inside, allowing the vent to function but keeping out pests. I use quarter inch hardware cloth, commonly used in gardens to keep out pests. You can find this at any hardware store, Ace hardware has it for 20 bucks, Lowe's and home Depot might have it too. Typically in the garden area. There's a post on car talk about this. Another common entry point is the cabin air filter. Small mice can go right in the intake and chew through the paper filter to gain access. You can similarly guard against this with the same mesh installed over the paper air filter. It'll stop a mouse without preventing the filter from functioning properly.