I have replaced the taillight bulbs on my '68 XR-7 with 6 of the LED from WCCC. The signals all work both sides, the brake lights all work both sides, the tail lights (head lights turned on) all work on the right, on the left side the one outer bulb is not lit. What may be the cause? Thanks.
Try switching bulbs around that may help in your diagnostics. Are you getting a good ground? Does this particular light work when the headlights are activated? What about the brake lights? Both the brake and turn signal work off the same filament sharing a common wire at the distribution relay located in the trunk.
Likely a bad socket for that bulb. Try swapping a different socket into that position.