Better look again. One red = POS battery terminal, one black = NEG battery terminal. That leaves you one wire short, which would be what you asked about. Where do I hook my tach. It depends on the tach. Most have an inductive pickup that snaps around a plug wire as was mentioned.
Is that a tach/dwell meter? It was probably supposed to clip to the coil on an old points-and-condenser car. Probably won't work on this outboard. Tach too old technology or motor too new technology, depending on how you view such things.
