Hello, I'll be helping you with this issue.
This should be fairly easy but will also be a little aggravating as it will require you to climb into the back from one of the front open doors.
The rear hatch panel should be held on by clips so, what you'll need to do is find a small thin object (preferably plastic to not damage the panel) and pry the panel loose from the snaps and hold it in place. After this you should gain access to the inside of the rear hatch and try to find the bar that is either broken or has come loose from the lock it self. You may need a long flat head screw driver to reach in and flip the lock over to "unlock" and then you should be able to open the door from the inside or the outside.
I don't know why, but your answer didn't post here until tonight here in the Pacific zone. Anyway, I've been prepared to do the very thing you suggest in your answer, but how do you deal with the single vertical stiffener screw that exists in the hand-grab depression in the bottom of the liner, only accessible once the liftback clears the body by a few inches. In other words, the screw in hidden and inaccesible with the liftback (and liner) in the closed position. I don't think the liner can be removed without damaging the screw mounting until the liftback is at least partially open. Thanks. I realize your reply may be delayed.
ok, I understand and you are partly correct in assuming that the panel may have to be partially damaged to gain access.
The only other way that I can think you opening that door without any damage is to find a borescope that is long enough to reach in and help you see where the lock is and then have slim-jim or coat hanger and bend it to help you flip the lock.
I have had to do this myself on a car that only had keyless entry because the manual locks were broken and the battery was dead, it took alot of time and patients but it works. Also purchasing a borescope will probably be cheaper than repairing/replacing a damaged door panel.
You can buy them at most of your major automotive parts stores or online(I would recommend ebay to be the cheapest place for this)
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Using a hole-saw, should I cut a hole horizontally through just the liner to the left side, right side, or centered on the latch mechanism? In other words, where is the unlatching cable attachment, or lever, or whatever located exactly on the latch mechanism. I would prefer to cut a minimum size hole that I can cover with a snap in plastic plug or colored duct tape (farmer's modern baling wire; and, yes I'm a farmer). Sorry for the delay. Been harvesting late hay.