Thanks for your post Falco, that helped when I couldn't get my tailgate open.
I always thought it was strange that there was no way to unlock the tailgate from the inside. I even had the interior panel off before and the third tail light removed and never noticed that lock/unlock button! I never know whether it's better to leave the tailgate locked or unlocked while camping. Unlocked: it would be startling if someone opened the tailgate while I was sleeping. Locked: to wake up with the engine on fire, and want to escape through the tailgate but unable to unlock it. Hm, I'm thinking of a workaround so I can access that plunger...
Anyway, I removed the latch mechanism from the tailgate. It was useful to put three pieces of masking tape on the connecting rods before undoing the plastic clips, so I could put them back in the same positions upon reassembly. I washed the latch thoroughly in bicycle chain degreaser (a detergent available at MEC) and applied grease to the latch, and oil to the mechanism pivots. And I cleaned the electrical contacts and smeared them with a little vaseline to prevent diaper rash (or corrosion from forming).
There's mention in this thread also of the metal tabs that attach the inner and outer skins of the door (and come loose over time). I had previously rejoined most of them with butyl tape (IMHO the best product for the job) but having run out, I mended the two remaining (since I still hear a rattle from the back) with some silicone leftover from sealing the shower stall. It's probably not high temperature enough for the application, but what the heck.
Got the tailgate all back together, with some minor whining about the plastic clips that attach the interior panel, and how did this stupid design ever become an industry standard. Oh poop.
Here's the only photos I took of the procedure, just a front and back view of the latch mechanism, in case anyone needs to see what they can't see while it's buried in the tailgate.

- TailgateLatch2.JPG (243.56 KiB) Viewed 5664 times

- TailgateLatch1.JPG (553.86 KiB) Viewed 5664 times
Hard to believe there's a solenoid somewhere in there. No harder, I suppose, that there's a lock button under the third tail light.