Steps:

• drill a pair of output holes to the right of the existing one in the metal end (just big enough to fit your desired red/black wires)
  • when drilling the holes in the metal, use a vacuum to constantly suck up any pieces, to avoid getting any inside the electronics
• drill a single hole (of the same size) in the PCB, from the bottom, between the two transformers (use a light from the backside to see where to drill it, it's RIGHT next to the long row of soldered pins)
• run the red wire in, tie a knot just inside (for mechanical strength), run it down between the transformers, through the hole in the PCB, and solder it to the backside (you can see there's a pair of big pins there... it's the one that's closest to the transformer)
• run the black wire in, tie a knot just inside, crimp a ring terminal on it, and screw it onto one of the existing screw posts along the side
  (you probably don't have a socket that's small enough for that nut, so use a small crescent wrench instead -- DON'T use pliers)