You should be able to pop off the rear cover. You will then see the + wire that goes the lamp. Find out which of the outgoing wires this is. Note the colour of that wire, and check if the bulb is still good (you can just pull it out i think). I would then apply + to this wire, and try to ground each of the other wires. When the lamp lights, you have already isolated the ground and one + wire. Next, connect two of the remaining wires to the engine as described in Fredrik's message. If it starts, you found the signal wires; if not, try the remaining pairs. The one wire left is + for the tach.

That's what I would do, but don't hold me responsible for any smoke you might see on the way... My tach had the red wires as described.