Yeah, I'm a little leary on wading in on this subject cause it was a heated discussion last year. That being said here goes. You need to have a ground wire from the sending unit to the battery ground. And a second ground for corrosion. You will have a voltage signal coming from the gauge to the sending unit. Your gauge is llooking for a ground to make the needle move, the sending unit id a variable resistor, the more current that flows thru the sending uniy, the more the nedle moves.
Here is a link to a typical wiring diagram
http://i3.photobucket.com/albums/y67...ingdiagram.jpg
Your wire colors may be different, But the voltage(red) wire feed the gauge, the ground (black) goes to the sending unit, thru the sending uniy and out to battery ground. The light(red/white wire) comes hot with your nav lights and the blue is the ground for the light. Hope this helps, comments welcome!!