My questions are where should check for the grounding issue for the tail lights/defroster?

Check the wires that go through the cargo door hinges. They might be visible with the door closed by looking down on the hinges. Wires get abraded, cut, and sometimes touch one another. The two sides carry defroster, third brake light, license lights and wiper motor power, door lock, and one conductor to ground the door itself.

Grounds for the tail light panel are near the lights. Pull the side panel in the cargo area and look at the light area. In my experience that area stays dry and has few oxidation problems.

The relay for the defroster is in the dash area, maybe behind the central control panel. Another post for that.

Good Luck,

Bob

:>)

PS Look down to Oct 26 for a post from Anthony2 about replacing wires through cargo door hinges.