Rain is undoubtedly getting into the boot, and until that leak is found and cured no amount of new locks will resolve the recurring alarm problem. Finding the leak is not going to be easy and I am not familiar enough with the cabriolet to know of specific weak points, so all I can do is give general pointers.
Look at:
-boot lid seal,
-side/stop light seals,
-holes in the boot floor (rain can be thrown up from the wheels).