Clean the attic first and remove any mice droppings and nests that you can find.
Finding inside of wall from attic.
When working from an attic space it will be significantly easier to access a wall space on the floor below by working either an interior wall or the exterior wall that is on the gable end of the structure.
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.
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.
All holes around the edge of the attic need sealing with mesh wire screwed with screws to prevent mice from coming back.
There are usually two openings far apart.
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.
Squirrels need to be trapped so finding their access point is imperative.
If you notice any of these signs try using a moisture meter which can be bought at your local hardware store.
Besides wall damage you may be able to hear dripping coming from inside the wall.
I ran ethernet from our basement to a couple of bedrooms by passing the lines through where plumbing goes to the attic for the soil pipe vent.
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.