FSM shows a grey coming from cavity 17 to the oil pressure switch. Any chance it's dark grey?
There's also a grey from cavity 20 to the coil.
Black wires come out of 7 and 8 for the back up lamp switch.
A black wire comes out of cavity 12 to the brake warning switch.
A black wire comes out of 18 and goes to the alternator.

Zooming in, it looks like it's coming out of cavity 20 and should be a grey wire with a tracer. Cavity 20 is next to cavity 24. It should be, in line, grey with tracer to coil negative (cavity 20), brown to windshield washer motor (cavity 19), black to alternator (cavity 18), and grey to oil pressure switch (cavity 17).
Next to those should be dark green with red tracer to horns (cavity 24), dark blue with a tracer to a splice that then connects to voltage regulator/alternator field/ballast (cavity 23), brown to ballast (cavity 22), and violet to the temperature sender (cavity 21).