Hey bobbyd...
There are a few ways to install a tach...1) Get a cluster for your year Civic from a junkyard with a tach..The wiring is already there to support one....or 2) buy an aftermarket tach.
I installed an aftermarket tach..Typical tachs have 4 wires....
red - connect to 12v (i.e. - to ignition, power to make the unit function)
white - light, connect to dash lights or similar
black - ground, connect to a good ground source
green - tach signal, I connected mine under the hood right next to teh distributor, however, the same blue wire should be inside your instrument cluster if you don't wanna go thru the firewall. Here is a pic of my connection...the green wire is from the tach.....and I connected it to the blue wire with a fold-over crimp type connector..I filled the connector with silicone prior to the fold/crimp to prevent corrosion.
