Check to see if the door open indicator light is on at the dash when the rear hatch is open. Same indicator as if you have any of the passenger doors open.
If not, then yes it could be a broken wire as Chuckles89 has mentioned, but it could also be the hatch latch itself. It may be that grease has worked its way into just the right spot that the latch does not make the electrical connection to send the signal that it is open.
You may want to squirt a bunch of WD-40 in there, then open and close-(SLAM IT) the hatch several times and see if that re-establishes the connection. This fix worked for me for a few days then I was right back to square one.
I'm guessing that you already realize the rear dome light is only intended to come on when the hatch itself is open, not the other doors.
Habanero has posts about this over on YotaTech. She discusses how to remove the entire back panel on the hatch and how to diagnose a bit further if you decide to get after it to that level.
