It could be a bad or unsecured battery ground cable between the battery and chassis or chassis and engine. Turn on the ignition switch to the on but not run position do you see an icon on the instrument panel representing a battery, if you do not see this battery icon the alternator is not seeing power to allow it to charge. Start the engine does the battery icon go out. Put a volt meter across the battery terminals with the engine off, at rest the battery voltage should be 12.6 volts if the battery is fully charged. Now get a friend to start the car while you watch the voltmeter, the voltage reading across the battery should read approximately 14.6 volts if the alternator is charging correctly. disconnect the plug at the back of the alternator at least one terminal should have power when the ignition key is turned to the run position (no power in off position) if not you have to trace wiring from battery to switch, switch to alternator. The heaviest wire at the alternator should have 12v all the time this brings charge current back to the battery to charge it.