What does the white wire clip to?

I wanted to give some info to this mystery. I checked my fuses with a test light to see if juice was going to them. First of all my fuse under dash that says instrument isn't blown and when headlights are on the test light lights up when connections at fuse are touched. Second if my lights are in off pos and key is turned to on all gauges work. When I turn headlight switch on gas guage drops to zero. Does that tell me anything? Trying to figure this out.

Carl