a OEM spoiler will plug in to the car in place of the window mounted 3rd brake light. If the spoiler is OEM, its possible the wirring was just never connected, for whatever reason. If it is aftermarket, I believe the wirring was usually spliced into the factory wirring.
Trace the wirring out of the trunk lid, assuming it was run, to verify weather or not it is connected to the cars wirring. It is possible the installer of the spoiler just never bothered with the wirring of the light. You may even have to remove the spoiler from the trunk lid to see if the wirring is connected or not.