There should be a liftgate ajar switch on the latch that is in the middle of the lower edge of the liftgate. It could be caused by a faulty switch, or the liftgate not being closed tightly. If you open the liftgate, and use a small screwdriver to flip the latch into the closed position (should be two clicks, I believe)it should turn the interior lights off. If this is the case, it sounds like the liftgate is not closing tightly. If the latch is in the closed position and the lights stay on, I would suspect a faulty switch.
