Look at that switch panel very carefully. That switch is reading 12/24V RUN/CHARGE. That means the switch in the RUN position you get 12 AND 24 volts to the motor foot pedal. The CHARGE position allows charging a 24 volt system through the trolling motor receptacle. 12/24 operation is selected at the MOTOR foot control, not by this switch. Since this boat is new to you, but otherwise used, only the original or prior owner(s) know if that system has been tampered with. This can be a very lengthy discussion trying to explain what you MIGHT have since we can't see your boat and the switch is missing. Three wire systems use black for 12/24 ground. The other two are 12 volts and 24 volts. To use this system, the trolling motor plug must have either 2 wires from the motor for 12 volt only or 24 volt only operation (not switch selectable). You have one or the other. If the troller has a three wire harness and three terminal plug it is a dual voltage motor and there should be a 12/24 switch on the foot pedal. You really don't need the RUN/CHARGE switch. Your charger is likely responsible for the battery issue or that battery had an issue. Two batteries in series are no different that one battery with many cells. A modern smart charger should not boil a battery. Boat manufacturers in more recent years are running two pairs of wires (one pair from each battery) to the panel at the bow. The troller connector would then have four terminals. The 12/24 voltage selection is again, either done with the foot control switch, or a jumper in the plug does the series connection for 24 volts on a 24 volt ONLY motor. Yes -- it's tough to wrap ones head around this but that's what "evolution" in boating has done for us.