Clean out the light socket as well - if there was a lot of that "anti-corrosion" yock in there, it can crap things up real good. I get rid of it as a matter of course - it's nasty, gets all over everything, and is of limited help.
You might need to clean the "strap" terminals at the bottom of the socket - I have found the Helix grey drafting erasers (you can get them as small cylinders used in electric erasers) to work well, since they're just slightly abrasive. Failing that, toothpase and an old brush works well - just make sure you rinse it all out.
I believe the grounds for the rear lighting can be found behind the trim on the left side of the cargo area - but don't hold me to it. It would not be a bad idea to disassemble that ground, sand the spot around it clear, sand the ring terminal clean, and reassemble. Coat with GB Ox-Gard when you've done that.
Given that this is apparently affecting only the one light, we can presume that this is a "local" problem - meaning somewhere after the front-to-back harness connection. I'd start with the light socket, and work slowly forward from there.
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