First you buy a good-quality 3rd brake light kit appropriate for the mounting angle on your particular car -- see the links below for examples. If your car's left and right brake lights are separate from the left and right rear turn signals (different bulbs for the brake lights and turn signals), then you just follow the instructions with the kit. If your car has combination brake/turn lights (the same red light burns steadily for brake light or flashes for turn signal) then you must run a new wire from the brake light switch to the rear of the car to feed the new 3rd brake light -- cars with combination brake/turn lights route power from the brake light switch through the turn signal switch and then from there to the back of the car; if you try to get away without running the new wire, the 3rd brake light won't work properly and you'll backfeed other lighting circuits with random results.
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.
Begin by removing the wiring harness from the back of your 2006 Avalon. 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".
Remove the wiring harness from the third brake light. Remove the third brake light retaining screws. The entire unit will need to be replaced. The unit is sealed and the lightbulb cannot be replaced.
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.
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.