I have the popular Planet Bike blinky for $20ish.
In the rain it often gets stuck on the 'on-constant' mode. At this point I need to open it up, disconnect the batteries, and then let it sit.
As of a more recent ride in the rain, it has stopped functioning all together. It worked when I got home, not it is not turning on. Is there anything I can do or do I need to replace this?