SharkSkin

Try swapping the light fixture with the front one. Pay close attention to the way the front one is wired, since you know it works.

Also, see if they work properly when hanging by the wires vs snapped into place. If they work until snapped in, you may have to dig a bit.

That bit of headliner on the hatch isn't that bad to take off. Sit in one of the back seats, remove the visors, and pop the clips with a long, thin tool like a mini-flatbar or even a steel ruler, if you don't have the right tool. You might want to get a dozen or so of those clips as spares, you will eventiually need them one way or another.