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You should be able to get to it by removing the inside panel on the liftgate. You will also need to remove the trim around the light and top of the liftgate. This requires you to drill out a few (six I believe) rivets. I replace mine with small stainless steel screws. This is not a difficult or long job.
As Don stated, the ballast (or transformer) is usually the first to go bad and easy to replace. The light assembly is actually a neon unit which helps explain its cost. The LED replacement is the way I would go as aquanaut suggests.