Grand Pisco Heater fan not working whilst engine is Running

[floss]

Hi I’ve been looking all over the various forums and can’t find anyone that has had the same issue but sorry if it has been asked before.
I have a GP 2014 with aircon and climate control on a touch screen in middle. If I turn the ignition on WITHOUT starting the car then the heater motor runs fine at all speeds as it should.
However when I then start the engine the fans stops working , ( not straight away it goes to half speed for 2 seconds then stops) the touch screen doesn’t change and is still responsive with the picture of the fan changing when I change the fan speed , but nothing from the fan its self .
Just before this the fan wouldn’t turn off and my wife took it into the local garage and the swapped the resistor block . I then asked them about it and the swapped it again .
I’ve put a multi meter on it and the fan seems to get around 11.5v on high speed and 7.7 on mid speed and 2.2 on low
Regardless of if the engine is running the feed (thick orange and black wires) to the resistor block still gives me a solid 12-14 volts even when ignition is off .

Many thanks for any help