I purchased a new trunk latch and installed it. Trunk light now works, so the mystery is solved. There is a micro-switch inside the latch.
So, I suppose, the order of trouble shooting a bad trunk light is: check bulb, check fuse, check or replace lock latch and finally look for damaged wiring or contacts.
Thanks to Google, I wound up here as I have the exact same problem on my 2012 Elantra GL. Even though it works perfectly fine, I'm quite willing to put in a new trunk latch to get the light working. I did notice that none of the external lights on the trunk itself seem to work either...wondering if this is connected to the latch issue as well?