Solder 3 wires to the 3 Points on the Backside of the Mainboard. The 3 Points are right under the Fan Connector on the other side.
Use some Tape to fix the wires on the Board. As you can see, i used the gap on the right to get the wires to the other side.
This is the upper side of the Mainboard, you can/should also use some Tape to fix the position of the wires. Also you can see, i glued the connector back on the board (no electrical connection).
Solder the wires to the Connector you glued on the Mainboard, or directly to the fan or any other idea you can come up with.