For caravan reversing lights, you have to use both sockets...the black cable one controls the main van lights, but the second grey cable has the reverse light connections. The reverse lights use pin 1, which is a yellow wire, and also pin 3 (white) and pin 7 (black) need to be intact too as these are the earth returns.
The other pins power the fridge and van internal battery. You'll also need to check that pin 3 (white) on the black cable plug and socket is intact too.
Forget about the indicator beeper - they have a mind of their own.....!
John
April 2009