Does the light monitor come on when you first turn the lights on? If it does, you know it's not the brake lights. If it comes on after the first time you press the brakes, then it's probably a brake light. You may further eliminate lights by using the blinker to isolate. (Remember to trun off ignition each time to reset.)
If you are positive that all lights illuminate (including 3rd brake, license (2) lights), it is probably a light that is barely intermittent, or a socket that is slightly corroded. Clean all the sockets, including the contact posts and the ground housing. You will find the culprit.