dam...
The alarm has been messed about with so-
Ideally you need a proper alarm man, who'll charge you to look at it
Its 20yr old kit if the rkp & cables are okay, a new battery & external sounder will probably be needed
Retain the wires & replace every item is my preferred option
Then you need to have a service agreement in place, as daft it seems they do require checking
To do things properly cost money (I doubt you have 20yr mobile. Tv or car as daily driver)