I give up...
Yep, no avail there.
the whole tailight assembly has been replaced.
It's a tin housing with a plastic lense.
And I checked for ground just like you said, I clamped my testlight onto the socket, blub in power on, and touched it to a good known ground and not a damn thing happened. I also used an old school bulb style test lamp. There's power coming thru the diodes the lightbulb would touch.
I also forgot to mention, the bulb will light when the brake light is on and when the turn signal is on, but nada for the taillight