I just started from scratch on mine.
I attached a new constant 12V to the battery terminal.
Found an existing chassis ground and added another ring terminal to it.
The grounded the negative end of my multimeter, and began unplugging connectors one at a time under the dash. I'd test each pin for 12 volts with the key, and accessory, and then off, to see which wire to take the accessory signal from. I then T'd off a wire into it.
And let me add, I know very little about wiring, and I didn't find this job too difficult.