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Quote: Originally posted by Kev537g on 15/3/2018

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Have you got a towing relay fitted between your car wiring and towbar irony

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Quote: Originally posted by Johnnie Boy on 15/3/2018

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Me thinks you will find it's a bad earth... just have to find it. The power is taking an alternative earth route and in your case through the lights and indicators.   

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Quote: Originally posted by navver on 15/3/2018

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Electric comes in two flavours for your car and trailer lights.
Positive and negative.

It travels from your car battery through wires as positive until it reaches your trailer light.

Having passed through the light bulb it becomes negative.

The negative travels back to the car battery through wires but often also travels through the car bodywork which is made of metal and conducts electric just as well as wires do if not better.

That negative part is called earth because grown up electric, like in your house, is connected to the soil or the ground so we call it earth.

So what they often do with trailer lights is use wires to get the positive as far as the bulb then a piece of wire to take it away from the bulb to the metal of the car. It then runs through the metal and another very thick cable connects it back to the battery.

They use one wire for the positive for each type of light. Side light, brake light, indicator etc but several may return through one wire.

An earth problem is likely because several things are not working right. The electric is getting there but cannot return properly, so it is probably going to an earth connection and then back into another light and then back through that other lights earth return making that light come on.

There will probably be a connection from a wire to your trailer metalwork which is faulty. If you find one give it a wiggle and that may make it work. Alternatively there may be a similar connection in the back of your car.

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Quote: Originally posted by Colin21 on 15/3/2018

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If it's a faulty earth, and I also suspect it is, the most likely place is in the plug or socket. I would try spraying some WD40 onto the pins and putting the plug in and out of the socket a few times. If that doesn't work you may need a new plug or socket.

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Quote: Originally posted by 664DaveS on 20/3/2018

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Definitely sounds like a poor earth connection. The plugs and sockets can get damp or corrode internally.

If you can borrow another trailor board to try it could show wether the fault is the trailor or car.

The 7 pin plugs are easy enough to split apart and check inside for corrosion or loose wires.
The blown fuse indicates a fault has occured.

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