Here is a mediocre reference shot from one of the Sony studios packs. It shows the wire going from the body, under the grip and then into the button ear, then from button ear to the Banjo bolt ear.
Here is another one showing the hole in the body of the gun. I THINK this is from the Spengler pack, but Im not sure.
The wiring on the wands in the movie were pretty simple. wire goes from the body, to the button, to the stobe in the barrel via the banjo bolt/red tubing. It looks like they would press the button on the ear and then the strobe would flash. The light didnt do anything.
My gun is a bit more complicated in that the light DOES light up, but my barrel light is inside the metal barrel and not in the clear part of the barrel. Here is the wiring for mine.
I didn't drill a hole in the body of my gun, I just used a dremmel to carve a groove between the bottom plate and the gun body for the wires to go through. I dont feel like taking my gun apart right now, so you will just have to visualize.