I recently purchased one of the above as I think it will be useful for checking lights/ATC the day before we go away without having to move the car close to the van which is rather difficult on our drive and in storage,especially when you are on your own.
If I check things the day before and they aren't working at least I have a bit of time to get it sorted.
Still check prior to setting off though.
They are a bit hard to find from dealers but found one here :-
13-Pin Extension Lead
They are sold by "another company" but they twice messed me around and kept me hanging on for months on the first occasion.
Before someone mentions it,yes I could have made my own up,probably more cheaply,but my wiring skills are not the best and I would rather have it right than fuse something and have to start sorting it out.