This is most likely
There could be a separate fuse for the lower brake lights , or maybe both lower brake light bulbs are burned out
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 fuse should be for all brake lights and tail lights if only one is out its a bulb
The third brake light is named for then Secretary of Transportation Elizabeth Dole and are called "dole lights".
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.
with a hammer
Check the bulbs, if o/k check the wiring to it
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