I bought some electronic gizmos, it is about 2" square, maybe 3/8" thick and plugs into 120Vac. They were at Costco, 3 pack for $10. I put them on a power strip next to a couple of washer/dryer pairs we had taken out and wanted to keep the mice out while we stored them until we got rid of them. Seemed to work, no mice in 6-8 months. I have never seen them for 12Vdc, but perhaps you could figure something out.. We use the peppermint oil on cotton balls (3 cotton balls in mesh sleeve with 2-3 drops of peppermint oil every few months). It helps, not sure it stops them all.