Now the autumn mornings are getting foggier and colder, a good windscreen demister is essential. My 2004 Citroen Berlingo heater blower was working on fan speeds 3 and 4, but the slow speeds were dead. The cause was a burned-out dropper resistor.
The way it works is this: There are three, wire-wound resistors, to obtain the fastest speed (speed four) the fan motor gets the full supply voltage, with no dropper resistor. A single resistor is switched in series with the fan in position three. Position two has two resistors, and for the slowest speed all three resistors are in circuit. The resistor block is in the air-flow to keep it cool - in fact it is in the same plastic housing as the blower motor itself.
The Haynes manual always cautions to remove the battery negative terminal before working on the electrics, but in this case the circuit only becomes live when you have the engine running. Set the fan switch to off, just to be sure.
In fact there is a little gap at the left-hand side of the glove-box where you can peek in and see the resistor. A small torch will help greatly.
If you push on the lever on the white plastic bit (closest to you in the photo) you can slide the white bit up slightly, and withdraw it from the blower. There is just enough wire to pull the resistor down below the fascia so that you can see to release the black connector. This is released by two little levers either side of the wires, you squeeze them together and pull the connector out. Reassembly (as they say) is a straight reversal of the above process.
But look carefully - there is a difference in the profile of the two white plastic housings. I had to modify the new one on the right to look like the old one on the left, with a pair of side-cutters and a small file. Otherwise, it just won't fit!
Don't be tempted to run the fan without the resistor in place - without the cooling airflow the resistor will get very hot . There is a thermal fuse which will go permanently open circuit if the resistor gets too hot, as would be the case if the motor in the blower failed. This is a safety device to prevent fire and should not be bypassed.
I am not sure why the new part had a different profile. It seems to work perfectly well, with four useful fan speeds and no nasty burning smells. I have messaged the Ebay seller to ask, and will post here any further information. The seller's description matched the model and year of my vehicle so, unless the seller made a mistake, it should have been the correct part.