My caravan side lights come on when I use the brakes,
(13 pin socket)

John, May 2014

This can be an earthing issue on the caravan brake light circuit if its lighting brakes and sidelights dimly.I`d suspect the electric is finding a better route to earth through the side lights.
It could also be a broken wire in the plug lead (curly whirly) if the lights all come on together brightly, look for crush damage on the plug wire, if its been trapped by the jockey wheel clamp it could easily join the circuits together.

spud, February 2016

make sure you have the correct bulbs fitted to vehicle stop lamp holders [dual filament double contact offset pin] 5/21 watt.

k, May 2014