c30 trailer socket/blinker error

hello together, my friend did not give any solution proposal of himself, except ‘must use the original cable set’ 2009 westfalia AHK with hunter box and cable set installed. worked perfectly until 2 weeks ago. depending on the caravan the blinker ticked. – the 4-light-weather turn signals in normal time – the outside mirror turn signals twice as fast – the relay clacks 2-3x so fast – tip3 times blinks gives only 2 flashes and the green flash arrow in the combii nstrulment stays on without it flashing outside you connect a trailer caravan or wheel carrier, the flashing functions are back to normal. has someone ne idea before I expand everything from b-pillar to suitcase room to check the cable salad? many thanks in advance gruss thorsten