Sorry, I dont have any switches or AC wiring diagrams handy so I cant answer that. If theres a blank spare terminal thats NOT for controlling the lights THATS PROBABLY IT assuming you have the correct switch i.e. one that has a LOW Charge and a High Charge and for Lights ON positions like the originals used on tractors with cutout relays and combination Low High charge and light switch controllers.
If theres an extra terminal and if its the right??? switch, an ohm meter on the terminal that wires to FLD on the genny would read like near 0 ohms to frame ground in High charge and around 2 to 4 ohms in Low charge.
To make her charge you need EITHER a correct light switch that has a good frame ground and its genny terminal wired to the gennys FLD post OR ELSE get a full fledged 3 terminal (BAT FLD GEN/ARM) Voltage Regulator correctly wired and toss the old cutout. Of course, if you simply attach the gennys FLD post to ground she will charge if all else and the genny is good but if you ran it that way (which is like High charge if a switch were used) a longggggggg time with no lights on you could potentially overcharge the battery