That is the location of the siren, the security module is incorporated in the pump that operates the locks.
When it gets wet it goes haywire, I guess it can go haywire for other reasons as well though. Unless you are lucky they can get pricey.
You have to remove the carpet and the passenger side trunk side panels and the rear trunk liner. You wold probably have to strip all the trunk pane from the car. Others have just cut the foam block covering the pump in pieces to a
Chess the pump.
Before doing this though see if there is actually any dampness in the trunk well where the pump sits.