I am sorry to have to point out that the eLearn diagram for the Heater Fan is one which contains some of the numerous errors contained in eLearn diagrams.
I am attaching my attempt at a correction below. I have modified the diagram connections to the fan speed switch.
Further as David says that his vehicle is a Rapido motorhome, it will probably have air conditioning fitted. A different diagram applies when air conditining is fitted. I will also attach a copy of that diagram, but as it does not apply to my vehicle (no aircon), I have not confirmed supected diagram errors relating to the connections to relays J022, J034, and J041.
On my vehicle, relay T12 enables the Heater Fan, and the Cigarette Lighter, while the adjacent relay T01 only controls the power socket. However eLearn does show two cigarette lighters on the diagram for fusebox B002???
I agree with the suggestion made by
Anthony489 of a possible relay fault.
To eliminate this open the LH cab fusebox.
Typically there will be two red relays in adjacent positions.
Remove the LH relay. (T12)
Move the other relay into the socket made vacant.
Switch ON ignition, and test the heater fan.
If a new relay is required be carefull. Do NOT buy one fitted with a diode wired across the coil. (Fiat seem to have ignored the Bosch standard for relay coil connections.)