reverse glow gauges questions

[Johnybeas]

k so I have reverse glow gauges I got forever ago off ebay and they look to be the same as the ones motorsport sells although i'm lost I can't get them to power up. I have the two wires plus another one i'm not sure what to do with it is similar to the ones that connect the gauges but it is open ended and looks almost like a prong ended piece that plugs into something.... anyone able to help me out???