Go find a cluster that will fit your year model car, and replace the entire speedo, gauge's, and such to where you do not have to hard-wire an external tach in your vehicle. In the long run, it will be better and less stressful. Otherwise, when you buy an external tach, there will normally be 4 wires to hook up. (I am basing all of this on an Autometer Tach) One will hook to your negative on the coil (mine was located inside the distributor, but I traced the wire that was hooked to the negative coil to the outside of the distributor itself and spliced into the wiring harness outside the distributor), one will go to a ground anywhere on the body of the car, one will hook to your in dash lighting (just wrap the wire around your dash lighting fuse), and one will go to your positive ignition switch (basically you can hook this to any fuse that comes on and stays on when you crank your car, for me it was the radio fuse) this should be all that you need to wire, but when it comes down to it, another cluster (found on ebay or a local junkyard) will be the easiest route! Have fun!