I really don't know how to make it any simpler... take out the cluster and look at the green plastic circuit on the back. There are copper traces laminated into it which weave their way through the circuit.... pick one to start with and follow it until it comes to it's end. While you follow each one look for cuts, deep scratches or burn marks or blistered spots. If all's well there then look at the sockets where the harness plugs plug in. The copper traces are exposed there where the pins of the harness plugs meet them. Look for any that are torn or peeling away from the plastic backing.
If all that fails, you can always pull the gauges and swap them into the other cluster. The only thing that the old cluster housing's (into which you'll be swapping the newer gauges) circuit won't have a connector for is the cruise control LED in the gauge face of the '91 tach. (Actually, there's a spot for it, but there are a couple pieces missing and the '87 wire harness doesn't support it either, so don't even worry about it. Just know that it's not going to work... just the LED that is.)
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