Both main brake lights are out, third brake light works.

Check the voltage or the connection.
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Multi-Cardiag M8 CDP for car and truck

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 whole assembly will pop out with a screwdriver (carefully). Then the bulbs can be reached. Just push the whole thing back into the slots when you are done.

There is no fuse for the third brake light that I know of, it is wired into the main brake light wiring on most cars anyway. Probably the bulb is blown.

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

Turn signal switch.

This is because your brake light switch has gone out. There are two sides to the switch one goes to the two main lights and the second goes to the third light. This is a common problem but is an easy and inexpensive part to replace. It is located on the brake pedal and can be changed in just a few minutes. Good luck and thanks for using FixYa!

I received your 3rd brake light flash controller-fully adjustable ez fit kit. installed on my 2002 gmc sierra, power coming into box, good ground and unit is not working. third brake light works good with kit removed. any help would be appreciated. thank you for your time.

I worked for a GM dealership,The problem maybe in the turn signal switch. The brake lamp circuit is wired into it to turn off cruise control.The third brake lamp is a circuit by itself.