Problem isn't with the '68 but with our '56 Suburban.
Sockets use an 1157 bulb and heres the wiring set-up and the problem:
* have three wires. Blue is dedicated to drivers side, red to pass, yellow runs to drivers side socket and is jumped to both the door mounted tail light and pass side socket. There is a blue and a red wire coming out of the brake light activation switch, up to inside the dash (mate with turn signal wires?) and come back out so I'm assuming that this is why these wires are each dedicated to one side.
* drivers side works correctly, headlights lights on or off. Longer filament works tail lights, shorter works brake and turn signal.
* Door tail light works correctly.
* pass side/headlights off. Longer filament lights dimly when brake applied. Longer filament works turn signal when applied.
* pass side/headlights on. Longer filament lights dimly. Nothing when brakes applied. Longer filament blinks slower when signal applied.
Lower filament on pass side bulb never works.
I've removed the socket from the housing and worked the bulb both ways (contact to contact) and have the same problem either way.
Wires/connections are all fine.