Look for the switches and see if they are the culprits.
If switches are left idle for a long time insulating corrosion builds up on the contacts.
Other than that I can't help much without being there to check it out.
A trick I used in the army was to drive a pin through the wires on the back of the switch to see if it made a difference. ie. do the job of the contacts
It depends whether it is a make or break contact to trigger the alarm.
That is my initial thought. There must be door switches boot etc they all need checking.
The main control box may be hidden behind a panel in the boot or behind a side kickboard. passenger or driver side.
Good luck. Without seeing it myself I can only give you limited help.
Cheers
