100% it will be the back cover
I bought some new back covers a few weeks ago and hey presto i have moisture in my lights 2 days ago
I also used a shedload of silicone so was sure this wasnt the issue - when i pull off the backing plate there was moisture all around the rubber gasket. so i pulled out the rubber then pushed it back in, although not fully. I only pushed half of it in then attached it back onto the light, this meant the rubber was better seated
2 days on, plus a load of rain i havent had any moisture build up