I would start by checking the wiring in the plug, you have a wiring problem and that's the easiest place to start checking. Do you have another trailer or trailer board you could try to check that the problem is with the car or caravan wiring?
Clean up contacts at both plug and socket, paying attention particularly to the earth. A spray of WD40 might help too.
If it was ok previously, unlikely to be wiring, but a check for strands of loose wire would do no harm
Good luck. Let us know result.
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