[BBM5]

Sounds like water getting into the system somewhere, most likely into one of the sensors, if one goes down the whole system stops working.

 

[Diny]

Alternatively, you can get cocktail umbrellas and tape them to your bumper sensors when it rains.

Seriouslyt, though, make sure they're pushed in all the way into the bumper. I find it hard to understand how water could be getting in from insider the bumper.