Consider getting a cat. I've tried to trap rodents before, and the only thing that worked was cornering them then surrounding the hole they ran into with sticky traps. Taking care of a rodent on a sticky trap is a sad occasion.

A cat on the other hand will not only catch some of the rats but the rest will smell where your cat has marked and reconsider living anywhere near your house (put the cat's litter box in the garage for best effect).

As for the rat thing, I've actually heard of this before, though not in a Prius. On one of those mechanic shows a guy brought in his car with odd problems happening and one of the 5 mechanics looking at it said, "You're going to find a rat in there." No one believed him until... they found a rat in there. They believed it got in through the outside-air ducts, but I thought they'd design smarter than that... I guess rodents really can fit themselves in anywhere.

Prius 2006, bought September 14: