As a rough guide, one of the wires should attach to the coil and one to the earth (the body). In theory, the amount of power given off from ignition coil determines how much power is going into the engine revolutions, so this can be passed to the rev counter, which is a simple dial powered not by a computer, but by simple voltage (on my car anyway).
By the way, the coil gives out 20,000 volts, and should NOT be messed with when the engine is turning.