Just plug the cluster in, even if it has no tach the wiring is there.
As for the harness no matter what model you get you can use that harness which may or may not require a little wiring depending on the model you got. Or you could use your stock Si harness if you still have it.
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Don't bother giving me "rep", I won't return it and I think it is stupid.
Gracie Barra Clearwater
Gyms in Clearwater, St. Pete and Port Richey
pm me if you are interested in trying a free class at any of the gyms.