Brake Light issue

Have you change the brake light switch on the back of the brake pedal Napa part # MPE SL330SB or # ECH SL330 about $8 to $12

If your third brake light works when you step on the brake, it is probably not the brake light switch. Have you inspected the bulbs? Some dual-filament bulbs can have a single filament burn out. If the bulbs are ok, look at the condition of the sockets.

I agree with Andy Y, but it does seem odd that both rear brake lights do not work. (Doesn't seem logical that brake light switch is bad, if third light works.)