Year/Model of vehicle?
Do you have a factory ammeter (1967 - 1975 model trucks) or a voltmeter (1976 and later)? Depending on the year, factory ammeters were protected by two 4-amp (glass) or two 5-amp (plastic ATO) fuses. It is not uncommon for one of the two fuses to blow and/or oxidation to develop in the fuse box, interfering with the electrical connection of the fuses. If you haven't already done so, verify all of the fuses and their connections before chasing wires.