That is one of the most common questions that I am asked about road light problems with caravan’s and trailers. Others are::
“Why do my side lights come on when I brake?”
“When I turn my side lights on why do the indicators glow dimly?”
“When I put my side lights on why do my reversing lights come on?”
“When I brake why do all my rear lights come on?”
The answer is fairly simple, and a lot of people will tell you “It’s an earth fault” So often said simply because they have heard someone else say it but really they don’t understand how it can be an earth fault. Trying to find a fault even if you know what it is can be frustrating if you don’t know a bit of theory behind it, but I’ll try to give you an insight into what’s going on. Lets have a look at a basic wiring diagram for the caravan or trailer road lights and their connections.