Aftermarket harness.

It boils down to original fit plug and play or custom made you have to make work.

Which harness, under hood or most likely under dash.
I bought one from Evans, for underhood on my '72 Challenger. The one that handled ignition, oil pressure and water temp, A/C, washer motor, brake failure.
It fit great, and everything works! Didn't realize how brittle my original wires were. Now the car starts every time!
They shipped it quickly, can't believe how high freight has gotten!