Lots of Honda Goldwing owners have a similar problem. Mice just love to build a home in the air box. Best solution seems to be moth balls.

My best solution - mouse bait/poison. I even make the cubed bait more enticing by smearing some peanut butter on them.

Also helps that we have a family of black snakes living under our back concrete steps. Keeps the vermin down to a minimum.

Just as a preventative measure every fall I place the bait/poison cubes where ever I think a mouse/rat/vole will try and winter over.

Must be working - no nest in my Pontiac Solstice (garaged for the winter), or any of my other vehicles.