I used a Cypress PSoC for the controller, because that's what I use at work. You could use just about any 8-bit micro, though. I conditioned the tach input with a resistor divider and a Shottky diode, then ran that into a capture timer. I powered it with a simple 7805 regulator on a Radio Shack protoboard, and stuffed it all in a little plastic project box. The box has some nice mounting ears on it, so it actually almost looks factory.
I used some spare Cat-5 to run the LED lines out to a smaller board with 6 red PLCC-2 LEDs soldered to it, which becomes a sort of remote display. That way I could make it very small and discreet, by hiding the brain box deep under the dash. I'm probably going to bump up the resistors, because the red LEDs are high-brightness (for tail lights) and get distracting at night. Green would probably be a better choice, but I had red on hand.
I tapped the green tach wire behind the cluster, just like any other tach or shift light. I can't remember if it puts out 1 or 4 pulses per revolution, I'd have to dig out the design and look. Power comes from the lighter circuit, which is fused for something pretty high, like 10A. I actually put in a terminal strip so I can add more junk later. That's stuffed in the center console, just forward of the shifter.