I bet it's the same as my '91, on the latch for the rear hatch there is a switch that gets pushed back when the door is closed. Over time it doesn't push back far enough making the door think it is always open. This means the light will stay on no matter if the light switch is in the 'ON' or 'Door' position. If the rear light only turns on when the rear hatch is open, it sounds like the switch in the latch works. If the light doesn't turn on when the switch is moved to the 'ON' position it sounds like switch on the light itself is messed up. Here's a picture of the switch on the latch. That black lead attaches to a tab at the bottom of that spring, grounding the circuit to turn the light on.
IMG_3414 by josh42_98, on Flickr