The wiring project on my large dollhouse was all my husband's "invention."  He used hard wire and drilled access holes through the walls up from the floor or down from the ceiling of each level.  In order to reach the room for the outlet, he then drilled through the wall into the "wire run" and threaded through the wire.  Each floor is self contained and in order to transfer the electricity from one level to the next, he installed copper plates in all four corners of each floor and ceiling.  When the 2nd floor is placed on the 1st floor, there is light!  Amazing!

This picture shows the 3rd floor upside down with some of the lines "routed" connecting the copper plates on each corner.  The opening in the center is the stairwell from the 2nd floor.
This photo shows 2nd floor "ceiling" with the lines "routed" to connect the ceiling fixtures with the copper corners.  The opening is the stairwell looking down into the 2nd floor.

My husband shown running wire into the "routed" lines and soldering the connections.
Doll House Christmas Decorations