I am assuming this occurs when connecting to the car socket.
According to my wiring diagram black is fridge earth Start again take the wire out re-fit and then clean the clean up the connecting pins on both the caravan plug and car socket. The other point is the wire may be damaged further along. If you spliced in using splice clips then may be badly connecting Scotch clips seem better from car spares shop.
found this in case you have no diagram.
Terminal
Number Function 7 core wire colour
1 Reversing Light Yellow
2 Positive Ignition key ON (Battery Charging) Blue
3 Earth White
4 Positive direct from battery Green
5 Reverse Sensor (Optional), usually spare Brown
6 Positive Ignition key ON (Fridge) Red
7 Earth return for Fridge (Terminal 6) Black
bob
April 2012