I think what Mel is talking about is a little black button type thing behind the brake lever. If you pull the left/right brake levers back, and look behind it near the brake cable, you'll see a little black plastic button. When the brake lever is not pulled, the button is pressed in, not allowing the brake light of the taillight to illuminate. When the brake lever is pulled, the button is not pressed in, causing the brake light of the taillight to illuminate. I attached some pictures of the button: a close up, one from underneath, and one from the side farther away. Maybe yours is stuck in, not allowing it to get bright when pulling the lever. Check this on both levers.
~Adam
