90% of the time it's the metal strips in the taillight housings.
They loosen over time and create gound faults that set off the lamp indicator even if the bulbs are okay. Pressing them back into place solves the problem for good.
Usually the metal track that houses the brake light is the culprit. So most will start a thread about the problem stating that pressing the brake pedal sets off the indicator.
Another common symptom is when you switch on the headlights after starting the car and press the brake pedal, all of the illuminated switches in the car rapidly flash on and off!
Hope this ends some grief to those of you who have been perplexed by this issue...