one of the prongs is for the guage lamp's power
what I mean to say is..you need a +12v supply to the guage(usually coming from a switch) plus the black(-) and yellow (I've usually seen orange for the fuel sender). The guage usually does have markings for the posts
Some power it from the "key on" power, some from "Instruments" panel switch, some from its own momentary switch so it only reads when you hold the switch on.
