Be sure you have the correct bulbs in both the taillights and the brake lights. Be sure the bulbs are seated correctly, and not partially twisted loose. An incorrect type bulb can cause this, and is, in fact, one of the common causes of this. I once changed bulbs in the rear turn signal and accidentally got the wrong bulb. Each time I put on the left turn signal, my dome light flashed brightly with it! Took me a few days to figure out what had happened.