Problem with caravan lights

Burtspieeater

We have recently purchased a 2009 caravan and had a tow bar fitted to our car (2009 CRV)

Just before our last trip I noticed a problem with the brake lights on the caravan. Individually all lights work perfectly but when the side lights are on, the brake lights are not bright enough to overcome the side lights -  I believe the brake lights are
working as the high level brake light is OK.

Do you think the problem is with the car or caravan?