There isn't a single cause for a battery drain on any vehicle. There are things that happen to a lot of cars that can be similar but you don't want to start with a guess. Testing is the best way to go and we use a low amps probe and a digital oscilloscope to directly measure the power being drawn from the battery. Sometimes we see a steady load and there are occasions that we see the load pulsing on/off. With the load that turns on/off you might be able to hear something turning on/off like the rear wiper motor on your tailgate. (common failure) With a steady load, you need a very accurate voltmeter (that can measure down to the microvolt range (.00XXX) and we measure the voltage drop across the fuses to find the circuit which is putting the load on the battery. Then we either move the current probe or if available use a second, or third one to isolate the circuit with the failure.
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