Pfeifer30

It is a Webasto Thermo Top C

I am wondering if anyone has a way to activate the interior fan without the vehicle running. I have an aux heater that has the option to function the interior fan so the windows will defrost in the winter. I have tried simply tying into the fan wiring, however this doesn't work. I believe the canbus system notices the engine isnt running and wont allow the fan to kick on. Is there any way to work around this? I have attached the schematic I have been working off of. Thanks,