Webasto supplementary heater
Hi Shezlee,
I found the answer on page 124 of the owner's manual, which advises that -
At outside temperatures below about +5C the supplementary heater is switched on and off automatically when the engine is running, according to the coolant temperature.
So I think the answer is that both the outside temperature and the coolant temperature determine when the supplementary heater operates.
Hope this helps,
John.