I have this pedestal mount Airguide tach. I want to use it with a Mercury Mark 58. It has four wires coming out of it. Two are for the internal light, the other two are red and green. After removing the back cover the red is labeled plus(+) and the green is labeled minus(-). Does this tach require a sending unit,or can it be wired direct to the motor? If direct, do they go to the mag and ground? If anyone is interested, I would be willing to trade this for the same tach in a surface mount.