My 90 kick didn't have a tach either... I went to a junkyard and looked at a tach from a 92, and took out the cluster with tach on my 89, and neither of them were going to work... I don't think that's the best method. Apparently the wiring is done differently and it is NOT that easy, but I could be wrong... The connectors were definitely different on mine, anyhow...
I ended up buying one off of JCWhitney for $29 and installed in on the steering column. It's backlit, works great, and looks very nice. I would have probably paid $30 for the factory cluster anyway.
My .02.