Help me chase some wiring - S13, black/yellow chassis wire

[Abeo]

I've been trying to figure out my idle problems with my 89 240sx with a S13 SR20 blacktop and standalone. Long story short, I've chased down the wiring and I'm not getting power on the black/yellow wire... instead, it's grounded. Looking at the FSM, it should be getting 12v key on.

Does anyone know where the black/yellow wire runs under the dash, between the large plug and the power source?

I've ruled out: the swap wiring, the chassis wiring from the brown plug to the dash plug. The ground seems to be coming from under the dash, which has me confused as it's stock under there. No other problems, no blown fuses.

If I can't figure this out, I'll just run switched power to it... but it's bugging me.

edit: I'll be looking into the starter relay next to see if it's the culprit