Tail Light Issues

The first thing I would do is check all the ground wires. Clean all terminals of any oxidation. Push the ground plugs on-off-on-off a few times to ensure good connections. Check inside all of the bulb sockets to make sure the terminals are clean, too. Use the pink eraser end of a pencil and scuff them a bit to remove potential oxidation. The rubber (not vinyl) erasers are slightly abrasive, but won't harm. Apply some dielectric bulb grease to the metal components of the sockets.