I killed three mice in my car over the last week.

I work in a rural area and end up leaving my car parked in the same spot for long periods of time. (I'm forced to leave food in there because of the working situation)

In 2014 you would expect an automotive manufacture to have figured out hot to keep mice out of a car. My 2003 Mazda has never had mice inside and that thing sits for weeks at a time without a problem.

As stated by the other poster it is likely the rear vents located under the back bumper.

I'm going to check with the dealer about it, but I expect I'll be fixing this one myself. Personally, I would think this could probably fall under a safety recall because it is not particularly safe to have mouse droppings inside your car. At a minimum it should be a TSB.

While parked at a busy campground, I had a mouse visit my car. I could see the droppings and it had chewed on a Pay Day bar that I'd left in the car. At this time, I did not know how it got in the car. Later, I found it came in through the rear quarter panel vents that are under the rear bumper cover. I set traps to catch it, but it had already left, probably the same way it came in. The rear vents are very accessible to mice, and even a rat or cockroaches, or any number of small critters. The manufacture should include screening on those vents.

This was one case where a mouse came into the car while parked at a trailhead. When I returned, the mouse was not in the car, but it's droppings were. And it chewed on some things. I tracked the droppings to the rear cargo area, and then down to the tire well where I saw tracks in the dust. The tracks led back to the left rear quarter panel where a large cabin vent is open to intrusion. It doesn't take much for the mouse to jump up in the fender, poke its head in the vent and come into the car. I spent a day on DIY vent screening to prevent it happening again. I don't know what other car manufactures use for vents, but I was surprised to find the Subaru vents so open and accessible.