Condensation in Caravan Lights

A very small hole drilled in the bottom of the lens will prevent a buildup of moisture.

Very common occurrence but never found it to be a problem,  it also occurs in many car headlights my own Volvo handbook says it is normal and will clear.

The light gets hot, the air inside expands and blows by the seal. The light cools and sucks in air from outside. The cool air has moisture in it, it condenses on the inside of the light. Repeat every day and you have a puddle in no time. 

You either totally seal them which is virtually impossible with external units built to caravan quality, or you do what Cyber suggests although I would put the holes in the lowest part of the lamp molding to let them drain and breathe. 

Thanks for your replies. Phishing's explanation makes sense especially as it is only one light cluster that is exposed to direct sunlight and therefore can get hot and exhibits the condensation. The idea of a small hole is also a good idea. I'd actually done a similar thing for the rubber cover on my 13 pin socket on the car. Another little job to do before we eventually get away late July.