Instrument cluster upgrade: oil pressure gauge question

[93w/t1500]

I finished the instrument cluster upgrade on the '84 Suburban and, with the help of the factory service manual, I moved all the pins around to their correct locations. I noticed their naming convention isn't the greatest (wires 39 and 11 controlling the hi beam and brake light to be specific) but I got it all to work. The only thing I couldn't figure out is the oil pressure gauge. The factory oil press switch for the idiot light had 3 wires running to it. According to the factory service manual there was another 0.5 tan wire that was used to run the gauge on the upgraded cluster. I looked all through the harness and couldn't find the tan wire. Has anyone else run into issues with the oil pressure gauge when upgrading?

I put a mechanical gauge in under the dash for oil pressure for now but I'd eventually like to figure out how the factory one works.