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Acording to the A1 schematic in the TM section of the forum the slave receptical is connected directly to the batteries...I think the slave plug goes right to the batteries. Check it with a multimeter. If it’s live with the switch off then yes you can charge thru it. You won’t find that in the first gen tms. It’s not on first gen trucks. Only later a1 trucks that have the batt disconnect box to the right of the battery box.