Typically and idiot light circuit has the generator/alternator output going to one terminal and the battery going to the other terminal. If everythingis working correctly the light stay off because it has 12V (approx) on both terminals due to the opposing currents. When there is a problem like the genterator/alternator not charging the current can then flow and light the idiot lamp.
If you can determine which wire is the generator/alternator output circuit and attach that to your new gauge you should be ok.
I would recommend consulting a schematic to make sure of the circuitry. I'm sure that someone a bit more knowledgeable will pipe in momentarily and fix you up

Good luck

Bobby