ideally you need an alarm man who been at it a long time or is experienced with working with old tat
if you don't have yours maintained there gonna be no one with experience of working with it, the bloke who once fitted them are either dead, retired, no longer on the tools or chucked it & doing something else
so you appear to have either got a fault on the tamper switch or the tamper terminals
if you've very luck you may be able to bridge the switch or rework the terminals with an iron.
also work all the dil switch up & down a few times, just encase the address is not right
personally I'd have ripped out & stamped on it years ago.