I used 2 hi-bright LED's and a laser sight from a builders level tool. I placed a LED inside the barrel on both ends facing inward so the light is even down the barrel, I then removed the laser tip from the level tool and fit it inside the front of the muzzle. All three where then soldered to the same wires, one going directly to the + pos side of the battery box and the other I took off the - neg side of the Hasbro circuit board.
You can also use cheap toy guns if you can find ones with nice blaster sounds, adding light to these are done roughly the same way, it's just a matter of fining what-polarity is coming off the trigger switching side, if it's + pos you solder the wired there and then the other to the - neg of the battery, if it's - neg coming from the switch then you put the other wire to the + pos side of the battery box. Here is a diagram of what I do with the rubies DL-44's, in this picture I took the neg power from the board but it comes directly from the neg on the battery box.