[ray_mcavoy]

They're dual filament bulbs so they can still light up without a good ground. Basically what happens is they backfeed through the other filament and find a ground through that other circuit. They're usually dimmer than normal when that happens.

Another thing to check is the turn signal switch. The contacts are set up so they can override the brake light on one side or the other as necessary for the turn signal. If those contacts are bad, it can allow the turn signals to work okay but not allow power from the brake light switch to pass through.