I have the MSPNP2, not the DIYPNP so idk about the wiring. On mine the o2 plugs directly into the WB gauge, and then a wire from the gauge goes into the stock wire for the o2 to go into the ecu. Stock location of o2 sensor right on the manifold, but on my stock ecu it runs fine, and then with the MS plugged in it runs rich. I'm also at a loss

Read the instructions

Per DIYAutotune's instructions, O2 on the DIYPNP mainboard is wired to 4N on the breakout board. This should be analogous to position 2P on the 1990 ECU connector - a red/green wire. If you're looking at the ECU connector, it's the bottom row, 7 holes from the left side. And I've already tried splicing the output wire from the AEM into that red/green wire. Never had any luck with it, getting Tunerstudio to show AFR other than ten, even with this setup. I guess I should note that while I did cut the stock 02 sensor off at its wire in the engine bay, that wire is still hanging out in there. I didn't cut it at the ECU, just spliced into it. Is that extra length of wire preventing the 5V output signal from getting into the DIYPNP somehow? Please forgive my extreme lack of analog electronic knowledge. I'm also happy to open the DIYPNP up again, remove the O2-4N connection, and wire the O2 straight to the DB connector, but I'd like to first understand why my current setup isn't working. Thoughts?

bump

I followed DIYAutotune's instructions when I built mine for my 2000. I just hooked up the stock narrowband input wire to the AEM gauge's output wire and that was correct. I ran the AEM wire into the engine bay, spliced the stock connector (from a dead O2 sensor), onto it, and then plugged it into the stock connector (harness side).

Also, it should be a red blue wire, unless you're blue green color blind :) . http://www.yorbalindamiata.com/images/wiring/90diag.pdf see page 4

I'll try the red/blue wire again, but I'm not getting why the AEM wire has to go all the way to the engine bay instead of just being spliced into the red/blue wire at the harness. Can you enlighten me on that?

Doesn't matter where it connects, just made more sense in my head to install it there. Let's say I needed to unplug the O2 signal for whatever reason, instead of ripping apart the harness again, I can just unplug a connector :)

SOLVED
 

I ended up setting up the DB15 connector to have 12V, ground, signal ground (in case I need it in the future), and 02, and went through that. Bam, Tunerstudio AFRs actually reflect what the AEM gauge is telling me. Calibration was slightly off but I fixed that quickly and now it matches perfectly. Now I can actually tune the car. Thanks everyone for all the help!