Provides a Convenient Electrical Ground and Power Source from the Vehicle. Plug the tool into the cigarette lighter or power accessory receptacle. You now have a ground and power source on the dash. The plug light will come on to signal the plug is properly connected and both power and ground are available at the posts. For power, connect to the positive (+) terminal. For electrical ground, attach ground wire from the instrument to be grounded to the negative (-) terminal.