I have just cured this problem on my 2001 with air con. Turned out to be the diode control unit built into the motor......Remove lower panel under dash on passengers side,this will expose the heater unit. Disconnect plug & single loose wire from blower motor, remove three screws & remove blower motor unit from housing. The diode pack can now be seen..... Disconnect the two wires from the diode pack to the motor & the plug into the side of it. Now remove the motor & fan from its casing, ( some are just rubber mounted push fit, some have single screw in end )
You can now remove diode pack held in by two screws.
These are available from Citroen but at a price. I managed to find a good secondhand one. You may be able to find a good complete blower motor, if so job should only take 15 minutes. Re-assemble is the reverse of dismantling.
Ken
July 2008