Luggage Compartment Open ?

Hi all in the forum !
I have an ongoing problem with my 2001 1.6 16v.
For a while, the door open lite was showing in the dash so I took it to my local garage. He diagnosed that the boot lock was the culprit. This was changed and the problem seemed to go away ( For a while).
I had wanted to get this fixed as I was going to get a Cat 1 Alarm fitted and didn't want the alarm going of in the middle of the nite with the boot switch giving it a false trigger.
The problem came back so I have not been using the alarm.
I decided that I was going to get the alarm set so as the boot was not covered in the hope i could use the alarm without it going of when it shouldn't.
I read somewhere in the forum that the main problem was that when using the rear screenwash, the water gets into the boot lock mechanism hence causing the part to "Play up ".
I wonder does it apply to heavy rain ?
What do I tell my mechanic to do ?
If anyone reading this can leave me their thoughts, soloutions or feedback I would be gratefull.

Mark.