The socket itself is a good thing to check. The connection of the ground
wire to the chassis can also cause the bulb not to work right, but that
usually causes the bulb to light dimly and the all parking light / tail
light bulbs to glow faintly when the brake is pressed.

Mike

Check the bulb failure sensor, it's a red can in the relay tray. I've had to
resolder the circuit boards in a couple of these when the taillights stopped
working.