There is no easy way of doing this other than checking all the connections for the RED wire in the 12N socket.
If the side and indicator lights work then there is not much likelyhood of the problem being with the earth (white) wire. So do all the checks for continuity on the red wire. Problems tend to develop in the car 12N socket (because there is constant exposure to water and damp conditions) also concentrate on the plug and 'A' frame conections.
Geoff deep down inside Wales, June 2010