I have a horse trailer which needs the wiring and lighting replaced due to corrosion over time. I have a mechanic who has offered to help me but I have been left with the task of purchasing what I need. My trailer has old 7 pin wiring, including a junction box which I assume is what allows me to have internal lighting.

My car has a 13 pin socket so I am thinking I would like to upgrade the wiring system. What would I need to purchase to allow me to have: Rear lights - side/indicators/brake/fog/reverse and number plate as well as internal lights?

My main confusion is will I need to change the junction box? As well as wondering if there will be any wires left over that aren't used (from what I've read they talk about 240v and fridge wires)? If I have wires that are not needed will this cause a problem with the lighting working?

3 Answers 3


While you see this as an opportunity to upgrade the wiring system of the trailer, I would nevertheless retain the 7-pin connector on the trailer and purchase a 13-pin to 7-pin adapter if you don't have such an adapter yet. There are such adapters for sale. You may need to occasionally use a 7-pin trailer with your car, and having both the option of 13-pin and 7-pin connectivity is good. It is almost surely cheaper to re-do the 7-pin electrics as needed instead of replacing it with a new 13-pin system. And, you already have all the connections made, so it's simply a matter of replacing old wire with new wire. In comparison, 13-pin system would require actually finding out how to make the new, different connections.

If you nevertheless go on to replace the wiring and light systems, you will obviously need a suitable length of wiring, new lights and a 13-pin connector. Here's the pinout of the connector: http://www.adelaidetrailershop.com/uploads/6/6/8/7/6687026/1922388.jpg?465 ...and here's what it replaces: http://findthingy.com/wp-content/uploads/2014/08/large-round.png

Are you planning to install fog lights as well? That's a feature that's not supported by the 7-pin connector. If your trailer has the 7-pin connector, it probably doesn't have fog lights, meaning you need some place to mount the new fog lights.


Besides some extra lines for reverse lights and such, the 13 pin connector has a line for a separate brake light circuit while the 7 ( and 4- ) pin connectors combine the brake/turn signal function.

So.. A large driver in your decision is what kind of lights are on the trailer. Common "small" trailers use a single bulb for brake&turn signal. You need to look what kind of lights you have or want to upgrade to on the horse trailer and go from there.

Aside, none of these connectors are intended for high voltage. That's kind of scary to think someone may have re-purposed some unused wires that way (!)

  • Do you consider a small internal light to be high voltage then? I have assumed that it was linked into the side lights circuit so that it provides power when the car lights are switched on? Sep 15, 2017 at 14:09
  • I was referring to your comment about 240v.
    – agentp
    Sep 15, 2017 at 17:20
  • All the info I can find relates to caravans, which have batteries and plugs... there is no info about horse trailers which is why I'm asking... Sep 16, 2017 at 17:55

I already have a 7 pin trailer to 13 pin vehicle converter that I used until the lights became unserviceable. The fact that both the current lights are rusted and the current 7 core cable is rotten determines that the whole system needs replacing rather than bit by bit. I am wanting to set up the trailer with led lights that have all the functions (side/turn/brake/fog and reverse). I have already researched and priced those.

I am thinking both of you have got the wrong end of the stick so just to clarify: my car is fitted with 13 pin socket, my trailer has a rotten 7 core cable and lights with very rusty connections. I want to replace the whole system whilst still keeping the option of having internal lights inside the horse box. No one has done anything yet... I understand a 12 core cable would give me four addition wires which are not associated with the rear light (a switched power source and an earth for that as well as a permanent power source and it's earth). I am asking whether there are any consequences to not attaching the permanent power source to anything?

  • you should edit the question with this sort of clarification instead of posting as an answer. (but no there is no harm in leaving a line unused.)
    – agentp
    Sep 15, 2017 at 17:23

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