how can i make my trailer lights work on my 57 plate transit tipper?

I have a 57 plate ford transit tipper and the trailer light socket does not work i have checked the fuse and cut off the old coroded plug and rewired a new one but still i have no lights. i have traced the wire to behind the glove box where it goes into a box with a red light on it this box clicks when you indicate one way but not the other do i need a new box and what is it called or is there some other thing i should be looking at

sb
September 2013