Usually something simple! (EB)
We had a problem with the tow-bar wiring on our motorhome in that there was no sign of life from the bike carrier lights, which meant we couldn't take the bikes with us to Malvern a couple of weeks back. After a calling the dealer and a little investigation they asked us to bring it over to their workshop for a more serious look.
It turned out that although the wiring and fitting had been done properly, the fuses had been left out!
Known in to us engineers as a basic error :-)