good solution:
Silica Gel - the small packets that come in shoes, and other boxed products to keep them dry.
Put one of each in each light - keep an eye on it mind - and take it out when it expands with the moisture. Don't let it get too big, or you'll not be able to get it out again.
I've not tried this method, but been told it works well.
That, or remove the rear covers etc for a while with the car parked up - let it breath. I did this and helped a lot.
Common
problem and not much you can do unfortunately.