Yesterday I got a dead animal smell coming out my vents. Today I pulled out three dead mice who had made a nest of tissues and car insulation on top of the cabin air filter behind the glove box. I found chewed acorns in the engine compartment and figured they might also have crawled into the engine air intake. Sure enough, when I opened the air filter box I found a pile of empty acorn shells underneath the engine air filter . I have a couple of questions. 1. The mice were on the upper side of the cabin air filter. This side is the one that faces the incoming external air. Nevertheless, I know the mice were getting into the cabing because they took tissues from the tissue box to make their nest and left a mouse dropping in the cabin. How were the mice finding a way into the cabin if the air filter blocked them? 2. Above and below the cabin air filter are two interior air recirculation vents whose flaps automatically open and shut in various configurations when the fan or a/c is running. The recirculation vent that is above the air filter does, in fact, provide a path directly into the cabin that bypasses the cabin filter when it is open. If this vent ever remains open when the car is turned off, I have the answer to question #1. However, it appears that both vents close when the car is turned off. Are these vents always closed when car is shut down, or are there situations in which the upper vent remains open? If this recirculation vent isn't the path to the cabin interior, what is? 3. Where is the air intake for the ventilation system? It looks to me like it is located somewhere behind the black plastic panel that covers the windshield wiper motors at the base of the windshield. Although this cover has screen across the front of it (on the car's right), the sides of the cover (next to the hood hinges) are completely open. There is a 4-inch gap at each end next to the hood hinges. This space is irregular and looks impossible to seal. If the air duct behind this cover has no screen of its own, then the ventilation system is vulnerable to entry by critters. We live in the woods, so mice will no doubt try to set up housekeepng again. The car has gone no more than two or three days without use since we got it in March. I want to keep mice out of the cabin, and I want to block them from getting into the ventilation duct and the engine air intake. I don't understand why the ventilation intake is not properly screened and sealed. My wife's Honda CRV has had rodents visit its engine compartment, but we have had no nests or dead mice in the vents because Honda didn't leave the ventilation intake cover open at the ends. Any ideas for preventing a repeat colonization of my Prius are appreciated.