Water in rear caravan lights

For the last couple of years I have noticed what I thought to be condensation in the near side rear lights on my caravan.  Periodically I take off the lens and dry them out. After recent heavy rain there was quite a lot ( half an egg cup full) in two of the three lenses. I dried them out again but this time sealed the join between the back of the lens and the clear front part with tape thinking that that was where the water was getting in. I also put silicone grease on the O ring on the bulb holder. Today after no rain and only 4 days from having dried them there is water again. It only appears to happen in the near side lights which can be in sunshine, the off side is always in the shade when stored. Has anyone else experienced this? It's baffling me now.