where does yellow reverse wire go onto lights?

I just got the grey plug 12s fitted to my car a (land rover discovery td5) and when i put the plug from caravan to the car the reverse light on the caravan was not working after checking bulb etc i had a look at the wire from the grey plug on the car i noticed the spark has not connected the yellow wire to the reversing light (he just taped it to the other wires) could anybody tell me if the yellow wire goes to the live wire on the reversing light.

Thanks