It could very well be a bad ground, but it would have to be a constantly powered circuit if it lights them up when the key is in the off position. Not many constantly fed circuits in an auto, brake lights, interior lights, cig lighter are the ones that I can think of off the top of my head. The brake light switch would be the first thing I would eliminate, check to see if it's faulty, or beening held in the energized state keeping the lights on. If it's ok, start pulling fuses one at a time until the lights go out. You will have found the power source, and narrowed down the likely cause to that circuit. That's how I would go about it.
