Not sure why, but the PO of this 86 22RE 4runner had no wire running to the oil pressure sending unit. I see no danglers or strays nearby so I assume it was cut closer to the harness.

Where would I begin to look for the wire origin?

From what I've been able to tell from my searching, it's a yellow wire.

Truck has the pressure gauge in the dash, not just the light. Larger OPSE as well.

Thanks in advance!

 

I didn't see the wire in the wiring diagram. Here is a link to the routing diagram.
http://snjschmidt.com/wiring/Wire_routing_3vze_1.jpg

 

You should be looking for a Yellow /black stripe wire

If it still has the stock engine in.