3rd brake light or rear brake lights?
Should be on same circuit as other brake lights
You probably have a faulty brake light switch
need more information is it the tail lights or the 3rd one in the spoiler or window
Is the bulb out on the 3rd Brake Light? Check bulb, harness attachment and wiring. Most likely its the bulb. See sources and related links below for bulb information.
if you 3rd brake light still works it is most likely a fuse. If none of your lights work usually it is the brake light switch located on the top of the brake pedal. Although there could be some odd wiring issue. Chances are it is the switch.
The fuse for the left brake light is # 5 and the right and 3rd brake light fuse is # 17 in the fascia fuse box
AnswerTry checking your fuse for your tail lights in your fuse box if that doesnt work try check for a short in your wiringAnswerI noticed on my 1990 did the same. Also found out it only happens when the TILT wheel is completely down. Try lifting the tilt wheel up and down with someone watching your brake lights. Have not pinned down the source in the column yet, but will keep looking!!!The brake light switch is probably the problem. My '91 had the opposite - the brake lights worked but the 3rd light did not. The brake light switch has 2 terminals - one for brake lights and one for the 3rd brake light. Replace the switch.
If it is the tail lights, it is the headlight switch. If it is the brake lights, it is probably the brake light switch. I really don't know how to replace the bulb in the 3rd light. change the brake light bulb first it may be shorted causeing the other problem
I'm not a mechanic / technician but if all ( 3 ) brake lights don't work ( assuming you have a 3rd higher mount brake light and all the bulbs and fuses are fine ) then it would be a problem with your brake light switch ( and also I BELIEVE if all the outside brake lights stay on )
First thing I'd check would be the fuse for the stop lights. If it is okay the next thing I'd check would be the stoplight switch located under the dash where the brake pedal is suspended.
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.