Do I have to have the engine running on the car to use the auxillary socket or should I be able to use the power from the car without the engine running ?
That is assuming your caravan is post 1998 or 1996 if its a BaileyJust to nit pick.
What happens is that the power from the fridge circuit operates the habitation relay which then switches off the caravan interior lights etc via pin 4 (green)and this wire is then used for battery charging en route.
So if you try to boost the output on site by running the car engine the interior lights etc will not work and this applies if you make a swift stop en route for the loo etc and the driver does not turn off the engine then all internal electrics including the electric loo flush and water pump(if you have a tank) will not work