i think the easiest way to find one mate would be to get the old multi meter out for a bit of trial and error. however be very careful what you hook up to and the current you plan to draw through the subsequent system i wouldnt put i high demmand on an external lighting circuit as they will almost definitely be controlled digitally and you could blow the control board.
i was planning on chucking a 240v inverter in the boot at some point with an attached/integrated 12v ciggy socket but for this i would run it's own power supply direct from the battery with it's own fuse (dead easy and cheap to do) for this you can use a car amp wiring kit they cost pittance and this method won't void your warranty and there is no worrys about it interfering with any other systems.
I can do that.But I was hoping that as there are a few people here that have fitted aftermarket tow bars which will have a permanent live, then there must be one readily available that someone knows about.
edit. I was mistaken, there is no permanent live on a trailer connection even on a split charge system so back to square 1. Edited by: burty