Just want to share my experince with the above issue. Hopefully it will be helpful for someone.
I had an intermittent issue with the blower fun not working (Citroen C5 MK III mK3 2010). It happened 3 times in 2 years, when was cold then after couple of days the fan started again. Today (4th time the fan is not working) I decided to fix it. The blower resistor (it is not resistor but electronic module - voltage controlled fan speed controller).
Resistor is located around passenger right leg (uk version of C5).
1. Remove the cover at passenger ankle.(one screw)
2 Remove the passenger glove box (plenty of screws) first of all disconnect the box door by removing two white plastic pivots in direction to the middle of the car(box hinges). -small mirror or phone camera may help.
3. Find the resistor which is located in the blower housing quite high - rather closer to the engine behind the air conditioning silver pipes. It has plug with 4 wires. grub the resistor housing, turn left and pull out.
1- GND (-) - green yellow-very tick
2- Signal - white-ish small diameter
3- Motor(-) - black very tick- motor power controlled by the module
4 - +12V red, tick - module power(not motor)
a) Blower:-resistor connector: short 1(green yellow) and 3(black) through multimeter 20A. start the engine - The blower should run on full speed - current should be around 10A.
b)Module: 9off the car) Connect 0v to pin 1 then +12V to pin 4, then a car bulb 21w between the pins 4 & 3. If bulb lit - module is faulty (possible transistor SUV90N06). Connect multimeter on diodes between 1(-) & 2(+) my has around 3 V open) - the bulb is lighting - module looks OK.
So I did all above, and everything looks good - when I assembled it back - it is working. Still do not know what was the root cause - I think the module cold soldering - so I tried to re-solder as many electronics elements as possible. Assebled again - still works
. I have some photos but I do not know what is the best way to attach them.