Typically for tach installation, there are 4 wires: The wire colors given here are for wires on the car, not for the tach. The tach wires may be different colors.
1). Switched power from the ignition - tap the red/green wire on ignition coil.
2.) Illumination - tap the light blue/red wire in the radio mount area.
3.) Tach signal - tap the dark green/yellow wire on the ignition coil.
4.) Ground.- find a clean, shiny, bare metal spot under the dash or in the radio mount area.