How difficult is replacing ignition (driver side) harness?
 

I looked it over, and it seems like it would be quite a project. There are wires running into the dash, behind the dash, somewhere behind the firewall, etc etc... Plus the headlight harness is part of it. Some of those wires I don't even know where they end up. What would I have to remove? Would I need to pull the dash out?

Is there a way to do just the ignition part of it, without removing the rest?

The ignition switch/harness is a separate piece (about 12" long) - BUT - as you may have surmised, where it plugs into the Main harness, THAT harness is an all-in-one rig running around in the back of the dash AND into the engine compartment - one big mass-o-wires....
Stu Aull
80 GS
Alaska

Right, so hypothetically if I wanted to just change out the ignition harness, could I simply swap out the 12" bit, or would I have to pull the entire tree? I'm assuming since I'm fuel injected, there's probably a wire that runs into the big harness and to the tach and likely ECU, so that ECU knows when to fire the injectors...so I'd have to get that wire.

I laid out my spare harness today, and it's scary looking. I figure it'd take me months to sort and trace all the wires, check them all, and then swap it in. I don't think I want to do that. At all.

I did it recently on my 12a.

https://www.rx7club.com/1st-gen-general-discussion-207/making-old-parts-look-new-again-862789/

Daunting to look at, sure, but if you can read wiring diagrams (I am half literate) it
shouldn't be too hard. I ended up gutting my harness and took out a good 3-4
pounds of connectors/wires/loom/etc.

I suck at reading wiring diagrams. Given enough time, I could probably do it, but it looks like even your project took a while. Purely for the sake of an experiment it's probably not worth it.

However I do want to figure out which wire goes to the ECU to trigger the injectors.