The "ground" Cletis is talking about is Chassis Ground, not that your switch legs are grounded. Check the bolt connections at the taillight housing, brackets, etc.
You could have the bulb filaments wired backwards. That would have the brake lights on as running lights and when you apply the brakes there is little change. Test the taillight wires (bulb) with a jumper wire from the battery and see which one gives you the bright stop light...
Mike B
