First you need to make sure the alternator is charging by checking the voltage across the battery terminals with the engine running.You should see 13.5-14.8 volts if it's working properly.If that checks out you will need to check for a draw on the system.To do that remove the negative battery cable and install a test light between the battery and the cable.Allow about ten minutes for modules to power down and then start pulling fuses one at a time until the light goes out or goes very dim.If the test light doesn't light you don't have a draw at this time.So the test may need repeated or you have corrosion in the battery cables not allowing all of the battery current to flow into the battery.You can check the cables with a voltage drop test but you will need a digital volt meter to perform the test.
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