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