You follow the directions that came with the tach. There are either four or five connections on the tach. 1) ground, 2) +12 volts when the key is on, 3) the pulse signal from the engine, and 4) the connection for the internal light. Some tachs have a separate ground for the internal light but most do not.
All of the connections except the pulse (send) signal from the engine can be connected to the same terminals on any other gauge. Markings of the terminals are: "I" = Ignition which is the +12 volt connection when the key is on. "S" = send which is the pulse signal from the engine. "G" or the ground symbol is ground. "L" = light connection.
There should be a gray wire in the wire harness coming from the engine. Check your control box just below the key to see if there is a three terminal receptacle there. If there is, that's a tach connection. It has the +12 volts, ground, and the gray wire connection. You will need the wire harness to use that connection. Otherwise, just splice into the gray wire.
