Steel wool is effective for mouse control, but it quickly rusts. A better alternative I have found is stainless or copper scouring pads available at the dollar store. If you carfully tease the pad appart, you can unroll it to give you a long "sleeve." Starting at one end, wrap the sleeve around the pipe (or wires). Continue wrapping adding a second sleeve if needed until the wrap fills the hole. Because of the continuous wrapping, the material stays in place more securely than if you just stuff wads of material into the hole.