Hello, Nicolas (and the other).
On the circuit there is this metal switch with four pins.
(I drew arrows around the switch on the image below)
Pins 1,2 and 4 are connected together to the circuit ground.
Pin 3 must be connected to one of the other three with a tiny wire.
Personally, I connected pin 3 with pin 1.
But it's the same with the other two.
It's pretty easy if you have a very precise soldering iron and a very good view.
Use a magnifying glass!
And of course insulated copper wire with a very fine diameter (~ 0.3 mm dia with the insulator)
I hope it can help.
[image|1525978]