They used to enter my van by climbing the tires (evidence: little footprints on rims in brake dust) then either the wires leading up the bat box vent, that I put there, or via the coolant hoses onto the radiator, then further following noses and using the small V-shaped openings at the sides and top where the plastic vent is screwed on. 
I taped shut the bat box vent (unnecessary with my battery type) and put plastic screen over the upper vent holes, held in place by Permatex Gasket 1-Minute Right Stuff. 
By the way, I removed the main bat box lid due to it no longer fitting due to the bat type I'm using, essentially green-lighting mice access. 
Bob

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On Jan 23, 2013, at 8:38 AM, "kimbrennan@mac.com" <kimbrennan@MAC.COM> wrote:

> On my westy they enter through the vent hole in the battery box. Yes, mice can jump, they can also climb surfaces that you might think are smooth. and the hole they need to get in, can be surprisingly small.
>