The wires for the temp switch are a bit harder to access, they're on the bottom right side of the radiator. I wanted to try out some stuff today but I couldn't reach them, I'll need to remove the air intake hose or something...
Here's a quote from the BX DIY guide at tramontana:
With air-conditioning: These models have two fans. In low speed, they are connected in series, so both should run (and because of the circuit, if one fails, the second fan stops running, too). In high speed one fan is always running, the second is switched on together with the A/C. The fan control switch is the same as with other 16 and 19 engines, the green wire serves the high speed and one of the blue wires the low speed (the other blue wire is earth). To check the high speed, touch the green wire to some exposed metal part: with the A/C on, both fans should run in high speed, without the A/C only one of them. To check the low speed, touch one of the blue wires to earth (if nothing happens, try the other blue wire): both fans should run in low speed. As the low speed is obtained by using both fans, there is no low speed resistor on these models.