I took delivery of a new Swift Challenger Sport 554 in September 2011 soon after picking up my new Hyundai ix35 2ltr CRDi fitted with tow bar and 13pin electrics installed at the garage. On plugging in the 13pin electrics all lights on the caravan worked except the reversing lights. I complained to the garage and after more work was told it had been resolved. It had not and so Tow bar specialist came to the site I was staying on to resolve. His eventual solution was to take one bulb out of the two reversing lights.(1 bulb worked but with two it would not) - not clever!
I subsequently found that all the lights had been connected straight off the car wiring with no relays. The car had one small reversing light but the Swift has two 21 watt lights. The problem was that the two bulbs overloaded the wiring on the car and so the lights did not illuminate. Solution- I gave up with the Tow bar specialist and fitted a 7 way relay (cost £23) so that all of the lights go through the relay and the load is not seen by the car switches or original car wiring. I feel that all modern cars should have one of these relays fitted and in future I will ask specifically for one to be fitted.
Hope this is of some help to others!