I would just carefully pull a harness from a 1984 G body with the same engine that is going into it. Sure, you can butt splice new wires in for things, but that method eventually leads to shorts and bad connections. Just pull a harness from a junked car with the proper engine family for what you are running and everything should be fine. I run a 1985 Grand Prix 305 harness in my 1985 Cutlass with a Chevy 350 and it came with a 231 V6. Just be sure the donor has the same gauge package relative to what you have or it may not work correctly ( basic or sport gauges, depending on what is available in the Regal and donor)