Joby
Here is a link to 13pin wiring diagram if you take a look under your a frame on your van to see if you have just one cable what has been split for your 12n & 12s plugs, if it has not been split, and you have your grey and black cables there, you have two choices, one is to strip the outer insulation of your cables already there, get a piece of heat shrink which will go over both cables, to form one cable, and then wire the 13pin plug on, the other way is to buy a new length of 13core cable & connect it back to the inside of the van with choc block connector
( mine was one cable what had been split )
You can sometimes pick up a pre wired 13pin socket on e-bay or are you just going to put both of your cables into the back ?
you have got to be careful that you don't get any stray stands of wire shorting across other terminals in the plug & socket ( there is not a lot of room )