Using your factory wiring can be a simple way to get a signal to your new gauge without running wires into your engine compartment. After you take out the instrument panel you will see the rectangular harness has numbers located at each wire.
By following the printed circuit very carefully I came up with the following identification and explanation of each wires function.
The first list is from an 86 K5.
2. Gauge lights
5. Oil Pressure
6. Keyed ignition
9. Water Temp
11. Left turn signal
12. Right turn signal
13. Seat Belt
16. Acc power
18. Fuel gauge
It has come to our attention that this is not universal, MrK5 traced the harness for an 84 CUCV, he came up with:
1. Bright indicator
2. Dash illumination lights
3. Oil Pressure low light
4. 4x4 indicator
5. Ground for dash lights on the left
6. Alternator #2 (truck had 2 alternators)
7. Also goes to Alternator #2 - ground maybe
8. Alternator #1
9. Ground for lights
10. Left turn indicator
11. Right turn indicator
13. Light below speedo, left side - mine had no bulb installed
14. Light below speedo, right side - mine had no bulb installed
15. Light below fuel level, left side - coolant temp warning
16. Also light below fuel level, left side
17. Light below fuel level, right side - Low Coolant
18. Fuel Gauge
The only two that we came up with that are the same are #1, #2 and #18.
Found this in reference to my plug as it was a CUCV military blazer: