It will be next to impossible to drill downward under the portion of the roof that slopes to the attic floor without a right angle drill.
Finding inside of wall from attic.
Clean the attic first and remove any mice droppings and nests that you can find.
Use interior walls only.
Squirrels need to be trapped so finding their access point is imperative.
There are usually two openings far apart.
For the mouse baiting in the attic and the basement should do the trick along with finding and securing the outside opening to ensure more won t find their way in.
To use it scan the device over the wall to find the area that has the highest moisture reading which will be where the leak is coming from.
If you notice any of these signs try using a moisture meter which can be bought at your local hardware store.
After all without a method to your madness you d just be hammering unsightly holes through your drywall in a vain attempt to cross paths with your wires.
Besides wall damage you may be able to hear dripping coming from inside the wall.
All holes around the edge of the attic need sealing with mesh wire screwed with screws to prevent mice from coming back.
If you need to find the wires inside your walls you ll probably want a non invasive way to look for them.