Cant comment on the 99, but on the 87 wagon, your lock assembly is right behind the hatch interior trim panel. To access the assembly, raise the lift gate and along the bottom edge is a plastic trim plate with 10 or so plastic screws and clips. Remove these and then gently pry out 2 push-in clips along each side of the trim (carpeted section) and that will allow the panel enough space to access the lock assembly and its workings. You can then verify any deficiencies or disconnected rods.

If you have to remove the trim panel completely, you will also need to first remove the 3rd brake light trim carefully and then lift the panel top (along the window ledge) up and away from it's retaining clips. This will remove the liftgate interior trim completely giving you full access.

If the electrical is intermittent you may have a worn wiring harness as it passes through rubber boots in the hinge area.

I would assume the 99 should be fairly similar

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Stable Mates:
1987 300TD 310K mi (Hans)
2008 Jeep Grand Cherokee OM642 165k mi (Benzrokee)