Has to be a ground issue with that left light, or the more common turn signal switch-wiring. Wouldn't hurt to make sure the hazard switch is not involved too. Anybody recall the 76 cutlass turn sig issues, where the two(upper and lower) regular taillights would drop out(one of the two) while turn sig or brakes were applied? Seem some newer Geo cars do it too. Step on brake at nighttime, and the whole side rear lighting would drop out. Not enough current got through ground circuit to feed all the extra filaments of tail and brake lights together.