Chevelles never had a third brake light since the Chevelle name was dropped after 1977. However, third brake lights were first installed (and required) in 1986.
1985
the third brake light is direct from brake light switch no fuse
Begin by removing the wiring harness from your third brake light. Remove the third brake light retaining screws. Reverse the process to install your new third brake light.
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The third brake light is named for then Secretary of Transportation Elizabeth Dole and are called "dole lights".
with a hammer
A third brake light is used to provide a deceleration warning to vehicles that can't see the left and right lights. You can learn more about these brake lights at the Wikipedia.
New bulb needed or broken wire
There are a few reasons that the third brake light is the only one working. One reason could be that there is a short in the wires of your main brake light. Another reason is that the light bulb has blown out.
Above the brake pedal. If your main brake lights dont work but your third center brake light does, then the brake light switch is not the problem. Its is the turn signal switch.