Today, on a hunch, he checked under the hood before heading out. Sure enough, something had chewed half-way through the air filter. He didn't have a spare, so we decided to use his truck to do the shopping instead.
We opened the hood of the truck, only to discover a decapitated mouse head on one side of the engine compartment. We then found a tail on the other side. And there was fur and goo smeared all over the inside of the fan housing. Someone picked a bad place to make a nest. Yikes!
We went out and bought a replacement filter and some metal screen material. When we got home, we discovered the hole in the air filter went all the way through now. The mouse had chewed the rest of the way through the filter in the few hours that we were gone. Which meant the mouse was probably still in the long, intricately curved intake hose.
My roommate cut the metal mesh to fit into the air box, while I got some paper towels, soaked them with balsam fir and peppermint oils and stuffed them into the air box. Hopefully the mesh will keep the mouse out of the engine while the scent of the fir and peppermint should drive the mouse out of the air intake.
I've been treating the inside of my car with balsam fir and peppermint and the mice seem to have stayed away this winter. The stuff's a bit expensive (about $15 for each bottle), but that's a lot cheaper than the $100+ to have someone clean out the air vents.