1) Mice can fit through a 1/4" hole. 2) There are myriad entry points into a car that mice can get through--for ventilation, drainage, and wiring. You're never going to close them all up. You can't close them all up; vents like the pressure relief flaps in the back can't be closed off by design. And, mice can chew (and must chew or their teeth will grow long enough that they can't eat). 3) This isn't a problem specific to the Prius or one that can be "designed out." 4) It's the luck of the draw. Most of us get lucky and don't have rodent intrusion. Just because there's a correlation with your specific preventative method doesn't mean it's a causal relationship. You probably just got lucky.