you might try a small bit of steel wool there. they don't care to gnaw on it, leaves an air flow [albeit limited], it's cheap to replace and it works in knot holes to keep birds [ and bees] from setting up house in out buildings ...

________________________________
 From: "kimbrennan@mac.com" <kimbrennan@MAC.COM>
To: vanagon@GERRY.VANAGON.COM
Sent: Wednesday, January 23, 2013 10:38 AM
Subject: Re: Wire Damage from Rodent(s)

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.