TOPIC: Tachometer question

Picked up another project with an 85hp '78 Johnson. Have to rewire the entire dashboard. What looks like a factory tach (matches the speedo) there are three wires coming from the back, the normal wires that go to the shifter/start/motor and an extra dark blue wire that was cut off. Was wondering if this is for a light?
Can't seem to find a schematic for this puppy.
Cal

There should be four wires; ignition, ground, sender, and light. Usually the light is a blue wire.

Correct you are, my mistake, four wires. All the others were connected but I didn't want to put 12v on anything before checking.
Thank you