I tried to put a tach in my Monte cluster, it already had the center gauges, but no tach. Anyways I think all the harnesses for G bodies were the same, with the wires, they are just all taped up inside. I actually found the white tach wire in my factory harness after I ran a whole new wire. But anyways the tach was acting screwy and none of the other gauges were working so I said F it and bought all new autometers. I would be willing to sell my SS cluster(s) if people were interested. One is like the one pictured and the other is the newer style, but the gauges are metric from a Canadian car.
Good luck with your gauge swaps.
