Hello and thank you for trusting us with your question. I'm DJ.
If the bulbs are lit up and even the smallest drop of water touches the bulb it will blow to pieces. The light bulb is extremely hot when it's lit, and the difference in temperature shatters the glass.
Water gets in to the headlight because air around the bulb gets heated and expands. The air is pushed out the vent hole. But when the light is turned off, the air cools and it sucked back in through the same vent hole.
The lens is usually cooler than the air, so moisture condenses on the inside of the lens and collects in the light.
You can replace the lights and still have the same problem.
The best way to resolve this is to use a hair dryer and cook out the lights when you see moisture building up inside them. I just set the hair dryer on a stool, point it at the light, let it run for 20 minutes then switch to the other light.
The heat will dry out the light. You might have to do this once or twice a year. I live in Portland Oregon, so it's a common problem here.
Please let me know if you have any additional questions on this or if there is something more I can do to help you.