A number of us have run led strips or blocks. The leds have just 2 wires so you can just splice into the 2 wires of the hatch light. Then just attach the led block/strip to the underside of where the cargo cover sits so its out of sight. You can even then run a wire from there to the other side and have lights on both sides and tuck the wire out of view behind the plastic I interior panels.
Hatch Nation #9
"2 by 2 Hands of Blue."