question moving light switch
Find a landmark that you can see in the bathroom and the basement, like a pipe coming through the floor. You can measure from it to the wall, hen do the same in the basement. Add two inches and drill up. You should be in the wall.
Or, take small drill bit and drill a hole at the edge of the base trim at the floor. Then push a wire through that you can se from the basement. When you find the wire, go a couple inches more and drill up. This is the method I personally prefer.