One other option and its easy and is more than a tach: Scan gauge II. Installs in 5 minutes, its plug and play. PLUS you get much more information than the stock gauges, air intake temps, coolant temps in degrees, speed, it stores and clears trouble codes, volts, and like 20 other functions. You get to pick your favorite 4 for the main display, and you can adjust the background color and brightness. At night I turn the gauges off and just drive with it, easier to see out in the dark. Worth every penny, it plugs in to your computer port at under the dash and you run the wire up behind the steering column and mount it with velcro tape behind the steering wheel or anywhere else you want it. I love mine. And it transfers to other vehicles too. The only down side about the tach function is that is will only scan the rpms every half second or second so there is a small lag time, especially if your accelerating fast. Yeah thats a big problem on this car!
http://www.scangauge.com/
Best price is usually on Ebay, if you get a second cord you can more easily move it back n forth between two cars if you want.