New to Caravans! - Dicky brake light!!

Hello all.  My wife and I purchased a Compass Omega 505 ( 2005). I subsequently got a tow bar fitted proffesionally and took my first trip. The passenger side brake light doesnt operate and since swapping bulb the problem has remained. I am no expert at all but wonder if / how i can track the problem to either towbar or caravan?

Any help appreciated!!

( be gentle - my first post!!)