The charge dash warning light is live on one side of the bulb with the key in the on postion. The outher side of the bulb goes to the regulator (internal or external)in the engine bay and to earth so bring on the light. When the engine is running at a suitable speed the regulator switch's over and puts a Pos supply on the wire to the dash. As the bulb now has a pos on one side and a pos on the outher side the bulb goes out. To test On the alternator find the small green or blue wire pull it off and short it to Earth the light should now be on if not Check the bulb has a pos supply on one side with the ignition on
On the back of the dash the metal plate the battery and oil lamps are mounted on is the positive this is insulated from the rest of the metal dash. below the fuel bits