Vauxhall Insignia headlamp levelling malfunction. service afl lamp

Only happens when it rains, once dry its fine. Motors in zenons works fine, but in rain light default to pointing down. Mot due again soon and it failed last time on this, and i got them to retest on a dry day and it passed! Is there a way to waterproof the sensors to stop this happening, or do i need to replace them? Is this stupid expensive if i do? Thanks in advance.