Are the circuits individually fused, Joseph, and if so, are they intact? If there is a fault with the light unit it is also possible that it/they may have 'blown'. The advice re cleaning bulb holders is sound, but the fault may have caused the fuse to 'blow'.
Do you have a high-level brake light? If so, is that working?
Joe H. is also correct in pointing to water ingress; we had the same problem with a 744 and resolved it in a similar way - a 1.5 mil bit in a hand brace and very light pressure, IIRC.
Regards, John.
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