sirtokesalot,
On that age of car I would suspect that there may be a broken wire in the rubber gaiter that the wiring harness for the tailgate goes through at the tailgate hinge area. Pop the gaiter off the body and the tailgate, gather it all to one end of the wires or other, and inspect the individual wires carefully. You may find the culprit.
If not there, you might take the light assembly entirely out of the ceiling headliner, take it apart and check that the switch isn't corroded or carboned up in the "door open" position. A cleaning with fine emery paper might do the trick.
Not sure what else to suggest if these ideas don't work out?