The 9 volt battery is for when the trailer is sitting on the dealer lot with no 12V batteries installed, the rear door can still be opened, once the 12V batteries are installed you can unplug from the 9 volt battery and plug into the supplied wiring harness that is also hanging down in the same box as where the 9 volt battery is
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19 F450 Crew Cab 4x4 Lariat, 17 Voltage 3970 Epic 200 Watt Renogy Solar System
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