There are two wires that connect to the alternator. The 16ga brown wire comes from the ignition switch, runs through the filament of the “Battery” light, to the voltage regulator. This circuit is “hot” any time the key is in the "RUN" position. This circuit energizes the voltage regulator which in turn excites the field in the alternator. Once the alternator has stator output, ground is removed from the brown wire in the voltage regulator and the “Battery” light goes off. The 10ga red wire goes through a fusible link directly to the battery positive terminal and is “hot” at all times. The alternator case is grounded to the engine via the mounting bolts. Ground cables between the engine and body supply the ground. Assuming each of these three circuits is functioning properly, (without excessive resistance) and the battery is good, insufficient alternator output or the inability to keep voltage constant, would indicate that the alternator/regulator needs to be serviced or replaced.