Numberplate lights fail when wet?

Hi All, I've had a good hunt around with no conclusive answer, I've had an issue with my Pre-FL VRS Estate for a few months now and I've been putting it off. When it's been raining my number plate lights don't work initially, then normally if I slam the boot or cycle the ignition after a short journey they work again. Has anyone had this before? Know how to fix it? I have suspicions that it may have something to do with the boot close sensor which I believe is in the lock.