99% of the time it is the heater resistor block, it is a bunch of resistors which gives you the different speeds. this is mounted close to the heater motor, pass side of the car. Usual cause of failure: water or snow inside the heater housing.
Switch problem? Cables disconnected?
Yes there is, but it only operates the high speed function.
It is the resistor, replace it.
try replacing the blower motor resistor
If the fan heater in your Citroen Picasso is only working on high speed, it likely indicates a fault with the blower motor resistor. This component regulates the fan's speed, and when it fails, it can cause the fan to operate only at maximum speed. To resolve the issue, you may need to replace the blower motor resistor. It's advisable to consult a professional mechanic for a proper diagnosis and repair.
do you mean the speed of the fan? if so, it'a relay generaly near the motor of the fan. If it's the heat who only came at high speed, you need to add some antifreeze.
Check the low speed relay and the resistor at the motor
now when you say replaced heater fan multi speed are talking the fan or the switch because ther is a fan speed resitor that can go bad and fan only works on high speed which can cause conector to get very hot and cause insolation on the wires to melt and cause a short in the harness making fan run constantly.
Check the resistor pack on the evaporator case under the hood
The high-speed setting on a heater fan may not work due to a faulty resistor, a malfunctioning fan switch, or a problem with the fan motor. It is recommended to have the issue diagnosed by a professional technician to determine the exact cause and appropriate solution.
The blower motor resistor pack is bad.
Blower fan inoperative or only works on high speed.