I recently bought a set of side marker from Barry and it had a wire (obvious ground) coming from the driver's rear, probably attahced to the tail light retaining nut. Is this a factory wire? Definitely has a purpose and will be used.

Doesn't look factory to me. And i'm not sure how much it would help anything either.

The issue I kept running into was corrosion right around the socket. Lights stop working, pull out the small wire brush, remove the connector, brush the edges good, plug it up and it worked again for a while.

Finally I added ground wires by soldering a wire directly onto the metal light socket pigtail and then ran it to a cleaned up bolt head slathered with dielectric grease. Not a problem since.

there is an earth wire on the LH rear side marker to tail light to provide earth for the tail light body. the RH tail light earths from the trunk lock rod cover.