I'm beginning to think that this (suggested by a few) may be the best option - along with a thin film of silicon or maybe even fmileto55's suggestion of liquid tape (which I've never heard of before) may be the next steps.

The other thing I am thinking is that over time the lights lose their water-resistance.

If anyone is aware of rear lights that are, and stay, waterproof out of the box, I'm still interested.

thanks for all the responses.