Mouse/rodent/chipmunk/racoon problems with vehicles parked or stored outside are common. ESPECIALLY where the weather changes from hot to cold at wintertime, they are looking to get shelter and nest. Last spring I found about 5 pounds of sunflower seeds in the engine compartment, and under the seats INSIDE the interior, and in the trunks of BOTH my classic cars, which are covered and parked right next to the woods and the birdfeeder.
On the plus side, all the seeds let me know right where to set the traps. I now use an electronic device that draws on the DC current to make a high-pitched sound that (supposedly) runs them off. Doesn't work nearly as well as a cat.....
Tip# 1: DO NOT place bait or poison, or your car will smell AWFUL for weeks, maybe forever.....
REB