The lights on a trailer are split in two.
The left hand side works off the Black wire which goes to terminal 7 on the trailer plug.
The right hand side works off the Brown wire which runs from terminal 5 on the plug.
The Earth system runs down a common wire. This wire is the White wire and goes to Terminal 3 on the trailer plug.
They split the load to both sides for two reasons. Firstly if one side goes out you still have the other, and also so there is not too much load on the wiring.
If the lights on the car are working okay the the fault lies in or around the plug. It is highly unlikely that there will be a problem within the caravan except damp in the light units causing rust and poor connections on the actual bulbs.
It is a common problem that because the plugs get very little use damp causes them to have dodgy connections. It is not a major job to get new plugs and replace them. There are plenty of wiring diagrams on the internet.
Hope this helps.
------------- Jan and Simon and Jet and Amber
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