Answer: It would seem that Chevrolet was a bit ahead of their time in the use of fiber-optics, and that’s why you can’t find any bulbs… there aren’t any. From 1968 to 1971, Corvettes had a series of indicators in the console and the dash to alert the driver to the status of most of the exterior lights on the car including the headlights, taillights, directional signals, and the license plate lamp. Light from the actual exterior bulb was transmitted through a fiber-optic cable to these colored lenses inside of the car. If a bulb was not lit, the appropriate lens in the console or dash would not illuminate, letting the driver know that there was a problem.
If left untouched the system actually works pretty well, but over four decades a lot of things can happen that can cause a malfunction. Parts of the system commonly break when disassembly of the light housings and associated parts are necessary, such as when the car is painted. Accident repairs are another common cause of problems when replacement parts do not have provisions for the fiber-optics, or the fiber-optic cable itself has broken. Parts are available, but many shops are not aware of this, nor are they interested in spending the additional time to repair something that they are unfamiliar with.
In some cases, the entire fiber-optic harness may be deteriorated to the point where replacement is necessary. This is a very labor intensive job because the fiber-optic harness is a part of the light wiring harness. There is a combination light wiring & fiber-optic harness for the front of the car, and another one for the rear. Either one or both of these harnesses may need to be replaced along with all of the related grommets, lenses and seals. Often this is done when a car is disassembled for restoration.
If you would like to attempt to make these indictors functional, I would suggest that you bring the car to a shop that specializes in Corvettes. They may charge you for an hour or two of labor to diagnose the problem, but at least they will know exactly where to look.
Let’s not overlook the obvious. Are all of the exterior lights on your car working? If not, the indicators are not broken, they’re simply doing what they were designed to do.