Trailer Pin9 Supply

[Salty Spuds]

I've searched the forums & downloads section but to no avail, so I'll ask the collective. I have a camper conversion based on a panel van with factory fitted towbar & electrics. AFAIK, Pin9 in the towbar 13pin socket is a permanent 12v feed from the vehicle battery. If I'd had a factory 2nd battery, Pin9 would be supplied from the 2nd battery. I now want to supply pin 9 from my leisure battery installed under the passenger seat, I know most of the towing electrics are under the passenger seat, so I'm hoping it's a simple case of finding the cable that supplies pin9 & swapping it onto my leisure battery busbar. Am I correct & if so, which is the cable from under the passenger seat to pin9?
Any guidance appreciated.

 

[mmi]

Short answer - brown 17-pin connector under left seat, pin 16 male?, red 2.5mm wire is coming from the trailer socket pin 9 (through a 8-pin black connector in left D-pillar).
EDIT: pin number

Longer but not complete answer - the red wire continues from the brown connector to the trailer controller/detection unit under right seat which is then fed from (both aux battery if factory installed and) main battery.