where is the ground located at the tail lights
in general the bulb metal side contacts the socket which is grounded to the housing which is likely grounded to the body thru the mounting screws which I ain't telling you anything you don't already know & if you take a jumper with an alligator clip on one end & clip it to a KNOWN good nearby ground then stick the thin pointer on the other end into the side of the bulb area to contact the bulb metal side that will elim or confirm a ground issue on that side of the vehicle.
