If it is just a shift light and not the electronics it would depend on how your rpm sensons switch works. I think they usually supply a ground so you would hook the red wire to a 12V source and the black to the rpm device (like a Dyna Shiftminder or MSD RPM switch). If you were looking for something that sensed RPM and turned the light on all in one unit you would have at least 3 wires (usually Red=12V, Black=ground, and Green=RPM input from coil).
Hope this helps.