That thought had occurred to me - I'll need to go to a standalone when I turbocharge the engine anyway but I really didn't want to go there yet. I'd much prefer to get the car running as-is and use it to happily pootle around town for a few years until I pull together the money and motivation to start the build-out for race engine duty in the white miata. It all depends on whether Jeff can figure out what the issue is currently.
Oh! On the plus side the sport brake calipers and rotors you sold me are installed in the car now and they work great! If/when Jeff gets the stalling-under-load issue sorted the car will be completely roadworthy. (Ok, I'll change out the soft top too, but that's prepped and should be a pretty simple swap-out thanks to having duplicate everything including a frame.)

Ran over to the shop yesterday after work and spent just under an hour poking at the wiring on the ECU trying to get the OBDII port to talk. Still no joy. Oh well, guess I'll appreciate the car even more when it actually does run properly.

 

Yeah looking at the pictures on a larger monitor, I'd be tossing that all in the bin. Not worth it. I'd be finding a decent condition chassis harness from the same year NA, then modifying for the NB engine, which is fairly straight forward. You've got all the plugs you need with the NB engine harness you bought.

 

Nice! Glad to hear man. I did warn you about the squeal of those pads right? Borderline embarrassing on the street, though I've learned to embrace it and just make eye contact when people stare at you like your car sucks (my current setup squeals too - just not as bad).

 

@curly I'm totally with you on that one. Further photographic evidence, now that Jeff has started doing a deep dive into the wiring:

Seeing Double!
[img alt="These are just sitting in the passenger's side of the engine bay, disconnected.
"]https://cimg7.ibsrv.net/gimg/www.miataturbo.net-vbulletin/2000x1504/20260314_145051_2c8fc9e88d61d5a3fde515fb683848e1fe 7935e9.jpg[/img]
These are just sitting in the passenger's side of the engine bay, disconnected.

So is this

This is apparently the rear oxygen sensor. As you can see it's zip-tied to the firewall.

What is this connector for? Is it important? It's by the brake booster right now, but heaven knows where it really belongs...

So there appear to be two (not one but two) nearly identical wiring harnesses leading to the ECU. Some stuff is wired to one of them...and some stuff is wired to the other! Jeff has been tracing wiring and sorting it out because he says, "It's only using, like, 7 of these wires."
He's sorted most of the issues, all of the gauges work except for the tachometer at this point, the car starts and runs, and you can even drive it on the street!
Remaining issues include:
1) It's pretty hard to start the car. You have to pump the pedal a bunch of times, floor it, let it crank for a few seconds, and then get off the gas once it gets going (obviously, or it would rev super-high because you've got it floored!)
2) The car still bogs down at mid-RPM range/medium throttle. Feather it and it's fine. Floor it and it's fine (and kinda fun!) It's just the middle ground that has issues. Jeff sorted the majority of the bogging down under throttle by unplugging and re-plugging the oxygen sensors. That appears to have reset/recalibrated/re-something-or-othered something in the ECU and it's much happier when you give it lots of gas now.
I was able to drive the car on the street for the first time yesterday - took it down to the gas station and filled it up for around 1 beeleon dollars thanks to gas prices in Phoenix right now. @Fireindc I didn't have any issues with brake noise in the drive - the hawk pads were pretty quiet, and even if they had screeched like the Porterfields I have in the white Miata, the red car looks like a complete rolling garbage can right now anyway - I am really looking forward to getting some time with the car so I can clean it up and hit the awful paint with some heavy duty cleaning compound. Oddly enough it looks Ok in pictures, but in Real Life...Nope. Here's a shot I took before the sport brakes went in.

Not current, but I never remember to take a picture. >.<
I've told Jeff once he gets the car roadworthy I plan on sending it home with him for a while so he can learn why it's so much fun driving a Miata.
Hopefully this heat wave passes and we get a little bit more spring-ish weather , though, since the car doesn't have AC and it isn't going to be fun doing anything with it until the temperatures get down to something a bit more reasonable.

 

That’s the AFM plug. On an NA6 the entire Harness is one piece, taillights to headlights. I’m trying to think what function they would’ve kept for the original ECU, maybe give it coolant temp input and still control the fan? Wire colors will be your friend here, you need to know if it’s OE or 99, but man they did this the absolute worst way. It’s a couple of wires to swap an NB engine in normally.

 

They've got a bare wire jumper in place so the fan is always on when the ignition is on, so apparently fan control wasn't a motivation.
The car was limping along well enough that I took it home for a few days shortly after my last post just to clean it up and drive it around a bit. Here's what came off of the car from a simple wash, and that's just the stuff that didn't wash away.

Got Dirt?
I tried to take the car out later that evening, made it half a mile down the road and the headlights died. I turned into a side street and drove home carefully, parked it in the garage and sure enough it was completely dead. Threw it on the charger and tested it the next day- 12.9 volts when it wasn't running and 12.5 volts when it was. I think it's an NB alternator - two small wires in one spot that disappear into the firewall, and one big wire that goes under the engine. I called it good and gave it back to Jeff at that point so he can figure out if the alternator is bad or if it's just not talking to the ECU correctly since I understand NB alternators communicate with the ECU instead of just obediently putting out power like NA alternators do. Jeff hadn't had time to work on it further when I took the Plan B car to the track last weekend.