Ok an aftermarket tach will have 4 wires, Red to Ignition, Black to ground, Green to Coil Negative, White to instrument lighting. Run a wire to the negative post on your coil into the cab.Do not hook the Red to a constant positive as it will charge the stepper motor and burn it out as it's getting no tach signal. Ground the black to any metal under the dash. You can either tie the white into the ignition or lighting to back light the gauge.