I had this last year and ended up getting a warranty replacement.
No matter what I did, I couldn't dry them out - even had some cloth on a piece of wire to soak up the water then left the lights on all the time but no joy so in it went - no problem since.
Also, check the access covers in the wheel arch haven't gone AWOL and are fastened securely - they are bu99ers for not clipping in allowing water to penetrate through the bulb opening