Been there done that..............except I converted an 85 NA Regal backwards to an 82 TRSC.
Here's what happened to me:
Converted NA motor to turbo using all the 82 parts. I swapped the computer and its harness over to the 82. The main body harness inside the car plugged right into it. Everything worked fine til I wanted the 82 dash to complete the clone. The main body harnesses connector that plugs into the back of the dash is wired differently between the 82/85. I thought I could just rotate it 90 degrees (which I did and it plugged in). That stunt made my 82 temp gauge flicker in sync with the blinkers, the green bliker indicators would not work.......I forget what else was odd. Upon further inspection I found the prob, that dash caused me to replace the main harness inside the car and the harness outside running to the lights, ign etc. due to the incompatability of the firewall connections. Since I swapped the engine side of the harness I had to install the 82 wiper motor as the connectors were different. Two of the 3 mounting holes lined up prefectly, but had to drill the third (easy). At this point everything worked perfect except no LOW speed on the wipers. I ended up installing the 82 ignition switch on the column to fix that. Even though it plugged in, the 85 ign switch was wired differently. Also the ESC module on the 82 is somewhere under the dash between the steering wheel and the radio (or was it behind the glove compartment). I believe your 85 harness will have it on the pass fender. From what I remember they are different, I had to get one from a junkyard. I swapped carb to carb, I haven't done a FI car. I have stripped several of the NA Regals and may have what you need if it's the same as a carbed car. If it's not too late, use wire markers to label/record everything you disconnect or to identify things that were not used. Both cars will probably be different (mine were)......it helped me keep things organized. Let me know if I can help you along with this project, i'm no pro, but I have been where your at. I currently have my 82 Clone, a complete 85 Type (99% stock) as well as my 87 GN, so I can take pixs of anything you need for a reference.
The more I think back when I did this.........if you can't make your 83 harness work under the hood with the 85 SFI motor.........you'll have to replace the body harness inside the car.........and that's going to drive you replace all the things I did including the dash. It is actually an easy 1/2 day job once you commit yourself to it and have everything ready. It's pretty much plug and play.
If you have the SFI harness in thru the firwall, it should plug into the 83 body harness for power and the SES light.....
