How do I change third brake light - 2003 Mercedes-Benz SL-Class

If some of the lights work then you probably don't need to change the light assembly. My first guess would be a burnt out fuse. Look for the fuse box and remove the cover. Usually on the inside of the cover it will have a diagram for all the fuses and you can find the one for the brake lights. Some vehicles have relays that are fried and these may be under the hood. Just look for a black box which contains electrical relays and other fuses. The last thing I can think of is a switch that is right above the brake pedal. When you apply the brake a small plastic or metal extends and this is what makes your brake light come on. www.lightdeal.co.uk

The brake light issue isn't likely the the brake switch (since it sounds like your other brake lights are working properly). It's likely a bulb in the third brake light. When the filaments get old they sometimes lose and regain contact as the car vibrates on the road. Change the bilbs and see if the problem goes away. If it doesn't, look for a bad wire connection around the third brake light. Also, have someone verify the third brake light isn't working from the back of the car when you see the alert, it could be a bad sensor.

change the globe and clean the connection and check the fuses

I had that same problem with my Dodge Durango, my back brakes ran on the same fuse which means all my brake lights woulda been out, so I changed the bulb and it worked fine after. However I had to lift my truck to change my third light, u might have to change from the truck or take off from back window

Replace the brake light switch located by the brake pedal. That will resolve your problem

there should be a screw on either side of the black plastic case, after that it is simple

Most likley the brake light switch. It has 2 different sides. One for the third brake light and one for the lower brake lights. It is inexpensive and can be changed easily. Good Luck

have you replaced the bulbs?

It sounds like you need a brake light switch. It is mounted under the dash on the pedal assembly. It is easy to change, just twists in place. Good Luck

open the trunk and look up along the top of the bumper and you should see two screws / nuts that secure the light assembly.