I got the converter lead when I got my car 1st car with a 13pin socket, but it was about 5 foot long, and so fiddly trying to make sure all the wires were loose enough not to pull, but far enough up not to drag, I got fed up with it... never did work out why it was so ridiculously long.
In the end, I terminated the plug wires of the caravan into 2 x 7 pin sockets, mounted on the A frame cover. On the odd occasion I need to tow the caravan with the old Volvo, which has 2 x 7 pin sockets on the car, I just use 2 Towsure curly plug to plug leads from the caravan to the car sockets.
For the cars that have 13pin sockets, I made my own converter lead using one end of 2 more curly 7 pin leads from Towsure. I took off the plugs at one end, striped off about 6inches of outer cover, then using heat shrink, brought them together as one cable, which I could then feed into a 13 pin socket.
The thing with the sockets on the caravan also means that when parked up, you take the leads right off, so there are no plugs hanging around, and if anyone does want to steal it, not only do they have to sort out the wheel, hitch and steady locks, but find a wire to power the lights before they can tow it away. :lol