Replacing signal lights on a 1992 sportster that has 2 wires on the original lighting to a bullet style turn signal that has 3 wires.

The two wire system is for a turn signal only unit. The three wire unit has a turn signal and a running light in it. Remove the turn signal lense and you'll probably see a dual filament bulb. The two wire system is shown as an "International" wired model according to my book. One wire is the "hot" wire (Brown) and the other is a ground or earth wire (black) on the bike. On the turn signal unit, probably all wires are black. You can use a VOM to determine which wire is the ground wire or you can just try hooking them up in different ways until you find one that works.