The lights on the back are grounded through the socket.
The wire is the Violet one that is grouped with a single White one about a third of the way down the left side of the diagram, going to the A through H connector, Terminal C.
The reverse connector terminal C is Violet wire going to the Backup Lt Sw (4 speed) in your case.
The other wire on the switch is the wire that goes to the same A-h connector, terminal D.
They missed tagging the color, but it's White on the other side of the connector.