winter storage and rodent control

[Marilyn Mason]

We have a couple of ultra sonic  rodent chasers in both cottages.  They just plug in to a standard 110 volt electrical recepticle.  
This was the first year we had mice in the cottage at 315 Rapid Road.  I caught a couple of mice with traps then got one of these units and have it plugged in over the kitchen counter.  I haven't caught any mice or found any of their droppings since plugging this thing in.  I haven't had any flying squirrels wandering around on the roof at night looking for entrance points either.
 I've been trying to find ones that work on 12 volt so I can put them in the 2 cottage garages as both have mice, chipmunks and squirrels lodging there in the winter.  
I don't want the buggers chewing the wiring off the outboard motors or bikes.  I've also had them chew their way into the heater of my truck.  The bill to remove the nest, replace the filter, and fan motor speed control wasover $250.00.  
I've checked around but can't find any units that run on 12 volts.  Any body know of this item.  Input would be appreciated.

Jim