There a little tab above the lead wires. That is the metal cover. That tiny wire connects that tab to the black (finish) lead. Wire the white and black leads to the switch, and add a ground wire to the pickup cover tab, and ground it. You can cut that tiny wire wherever you want; just break the connection somehow.
Are you heating up the tab, and adding solder to the heated tab? Or are you adding the solder to the iron? Add solder to the heated tab. If that doesn't work, use soldering flux. If that doesn't work, rough up the tab a little with sandpaper.