Maybe the easiest place to begin is at the relay and fuse panel, underhood, next to the battery. All the fuses have test probe spots on their tops ...you may have never noticed them. With the engine idling, first read the full battery voltage from "ground" ... then read from ground to each of the fuse test points. All should be a the same, exact voltage. Anything less, even a tenth of a volt, is a reason for closer investigation.