After lots of searching, dead-end upon dead-end I finally managed to acquire all of the parts I needed to wire up the EDM leveling switch to my euro headlights. I realize this is going to be a super-niche DIY because they were rare to begin with and even fewer are changing hands these days. Some dude cranks out the harness for $250 but, I do electronics and instrumentation work to pay the bills in between project cars so, I'd rather just do it myself and learn something in the process. Also I've been itching to solder something.
Here are the wiring pinouts that I used, from the S14 SR20DET manual in our very own Service Manual Library
Sweet! That's the easy part, now I just needed the connectors and the switch. These things are pretty damn hard to come by and people seem to want big money for the switch if you can even find it as a Nissan part. A little digging and I found out that Subaru bought some of their switches FROM NISSAN around the same time and this is WAY more popular and readily available in that camp. Goofy looking 'Subaru' logo there
You're going to want to search for a 2002-2005 Impreza headlight leveling switch. It'll have an 8-pin connector (2 are actually fake nibs but I'll show a picture below.) They come up on ebay for a reasonable price all of the time.
The connectors are a lot easier. Here's a link where you can get them.
These are the male connectors for the headlight assemblies so you'll need 2 of them.
Make sure you get the option that works for you. The default is just the shell so you'll need a pigtail (connector with wires and pins already installed so you just have to solder to the wires) or the whole connector kit with pins if you want to do it right and have a nice clean looking harness.
And the connector for the switch
You'll just need 1 of these.
Now that you've got connectors and your pinouts, you're good to go! In case you don't regularly read wiring diagrams I'll do a simple pinout of how to wire it all up:
What I did was source the 12V on the headlamp harnesses from the fuse box in the engine bay. I just used a simple add-a-fuse. This is going to draw the most current of the two 12V supplies in powering the actual motors in the headlamps. I think 18 gauge wire should be plenty.
The OTHER 12V from lighting is the arming voltage. I drew that off of a common wire I tapped off of in the cabin, in my radio harness. This one you only want to come on when your headlights are on and it's going to be supplying the power to the 'location' part of the leveling circuit that tells the motor to go up or down until it reaches the commanded position. This one won't be carrying much current at all.
The last part is the ground. I went ahead and supplied a common ground wire between the switch and each of the headlamps, then terminated it at the passenger headlamp on a standard ring lug terminal. This way I only have 5 wires that are running from the cabin to the headlamps (ground, and the 4 wires associated with positions 0,1,2 and 3.) Less to fish through the firewall that way.