I show #8 wire is the positive for the ( deleted ) amp gage and #9 is the neg for same. These were not used when the military switched the trucks to voltmeters ( and the 5 wire inst harness )
I may need to clarify "not used" as in with the amp meter, you may need #8 for the voltmeter and maybe add a double plug to supply the 5 wire, you'll have to check behind you inst panel and see which# wire on your old harness runs to your voltmeter just to make sure.