Anyone know if you can tap into the wire off the ICM for the tach? That would be the easiest place, considering there won't be anything on the cluster wiring. Maybe a tach isn't the right thing.. perhaps just a volt meter is required to tap into the ICM wiring? Padgett seems to be the guy who's messed with the ICM wiring the most. He's probably got the answer there.

it's been a very long time (say... 4 years) since I've owned that year of car, but if I remember correctly it will be a white wire on the ICM that is your tach wire. Not just any tach will work though because if I remember correctly it's only a 6 volt wire, and most tachs operate on 12 volts. I was never able to get a tach working on my 88 LeSabre.