I think it's likely the switches are two position single pole changeover being used as on/off so a third pin redundant. A multimeter set to continuity will confirm. I can only repeat the black and green wires you have correctly identified should provide a shared ground feed (black) to one leg of each seat pad with the other connected to it's own switch. A further shared feed from the green wire should run as power to each switch.

The chassis loom schematic is just that with it's terminations to/from being shown as labelled connectors to discreet components not shown.