I'm 98% sure it connects to ground on the screw that holds the switch up. There is a little washer type thing that you screw through, and the wire connects to that.
Rather then soldering two wires on that one terminal. Hit the side of switch casing with sandpaper/file to roughen it up. Then fold the ground terminal over so it touches the switch casing. Then solder your ground wire in. This way your only really making one connection.