What does the manufacturer say? Don't they sell a harness?
Tachs need ign power, ground signal and have a light for night boating.
Actually, you can check the pins to the light bulb. That will tell you night light and ground. Then it is 50/50 on the other two pins.
Don't overlook those 12-24 bolts. The look like the connections on the older tachs that used ring terminals. Bottom left would be signal, top right is lights, middle two are ground.