I'm assuming that you are referring to the brake lights on either side of the tailgate / trunk.
Open the tailgate / trunk. Remove the two screws that hold the light cluster in place.
You would expect it to slide free as you pull it toward you - it doesn't. That's because there is a pin that protrudes from the light cluster that seats in a hole in the side of the car. Pry it loose by running something flexible and plastic (to avoid scratching the paint) around the upper edge of the light cluster housing. I used a plastic kitchen implement. It should pop free with some wiggling, allowing you to draw the cluster out of it's place.
Replacing the bulb is easy after that. If you're asking how to replace the eye level unit, I don't know, mine has never failed. Good luck with your repair!
Its a whole led unit replace whole deal not bulb
'P'arking or emergency brake
The 1999 Mercury Sable license plate light and the right side brake light are on the same fuse. If the fuse was bad neither brake light would work. The problem probably is burnt out lightbulbs.
Either your parking brake / emergency brake is on or the BRAKE FLUID LEVEL is LOW , in the brake master cylinder located in your engine compartment
Put this question into google, and you will find many reasons and remedies to this problem. Ei...brake light switch, brake light bulbs, including the upper brake light bulbs.
This is usually cause by a stuck brake light switch on the brake pedal. Replace it.
Why ? Just replace it.
My auto lamp light sensor is not working on my Mercury Sable 2002. Why?
Just unscrew it.
The brake lights on a 1998 Mercury Sable are changed by removing the retaining screws, pulling the light assembly away from the vehicle, and removing the bulb. A new bulb can then be installed and the assembly put back in place.
Just unscrew the rear brake light, turn out the socket, and replace the bulb.
check your brake lights if they not worhing replace fuse or brake light switch