Check out this Lego custom minifig head, the eyes and mouth were cut out and an LED inserted in to the  Lego custom minifig head, the LED is then powered by a small battery that would fit inside the torse of a Lego custom minifig, it’s very effective and could really enhance a Lego vignette, but it would also limit how much you could do with the Lego custom minifig after this modification.