Neil,
I got my information thanks to Steve W on the forum last Novemeber. I have copied Steve's instructions which are in more detail than mine............
If you have a 13 pin socket already fitted to your car, all you need is the plug to fit, an appropriate lenth of 13 strand cable, ( I used 3 mtrs for car and van, a strip of good quality screw connectors. I cut the 12N & 12S plugs of the ends of the wires and pushed them up through the floor into the van, located the cut ends under bed box. Removed the outer covering from both cables and leaving plenty of cable to work with, cut and join the different wires one at a time.(I always solder the cut ends of wiring for a better connection) The 12N wires match the colours in the new cable. The 12S wires need to be transposed on to the wires as they are arranged in the old plug. ( These circuits can be found on Western Towing web site ) I then covered the connecting strip with a junction box and "Voila"
On my van I didn't cut cut the cables in the bed box because I found the wiring on my van split in two different directions beneath the van and went one side to the fridge and the other side to the fuse box etc..So that's why I cut mine towards the front of the van. Suggest you have a good look at the cable runs before you cut them.