12 volt grey socket middle pin over heating

Drobbly

Nissan x trail 2004 and Bailey Unicorn. Had both from new car fitted with both 7 pin sockets the caravan has the new 21 pin socket so I am using the converter. Middle pin has started to melt the socket. This has been giving very faint brake lights and making
the front caravan side lights come on when braking anyone got any ideas...