The issue with the rear window defroster has been solved. I will highlight the steps needed to resolve this below through pictures.
First, the part I was dreading, taking the entire rear hatch apart.
What the rear hatch looks like with all the plastic panels removed. I did notice most everything is connected to the door metal for negative, but then I also noticed the same central negative connect point also had another wire connected that would run back up the wiring to the front of the Leaf, so I'm guessing this is a redundant negative connection since the metal to metal connection is on a rotating hinge. To avoid sparks and / or welding while moving the door up or down, this was probably necessary.
Finally, the moment of truth. The connector has broken off from the soldering points.
Maybe I am slamming the rear hatch too hard? Well, I did check to make sure the break point wasn't because of melting, looks more like a sheer break. Maybe years of slamming the hatch or not enough solder at the factory? Either way, I have plenty of Solder I can use.
After carefully soldering the connector back into place and adding enough extra to hang a potted plant on
, I gave it time to cool down and set properly. Followed by some simple tap and movement test to make sure it wasn't just going to break off again immediately, everything looks solid.
I plug the connector back in and do a couple of test with opening the hatching and slamming it shut to make sure it was going to stay, everything is still solid. I checked the other side to make sure I didn't see any cracks or if it was loose. Everything was quite solid on the other side. If it ain't broke, don't fix it.
Next, fire up LeafSpy to see if it is actually using power to heat up. I first start with every possible accessory turned off in standby mode to get a baseline power reading. Then I switch on the rear defroster and immediately see the proper power draw. I give it some time to heat up and feel around the elements by hand. Everything is getting nice and toasty, hot. I switched the Leaf to "drive" mode and let the rear defroster run for another 10 minutes to make sure nothing was going to melt or burn; just in case.
I believe my primary issue is solved. I put everything back together in the rear hatch. This still doesn't explain why my mirror heater on the drivers side does not work, but you know what? I will take the win for today on the rear defroster heating element. I'll save that mirror heater for another day.