Car alarm issue

Hi everyone,
My Fiat Car Alarm (the original one) has stopped working, although the Infrared handset still works for the central locking. Is there somewhere a hidden battery in the alarm unit (or a fuse) that I need to replace (after 23 years...)?
I‘ve also checked the car alarm switch, it is set to „on“.
Any hints? Thanks for helping.
Uwe

When I bought my coupe 12 years ago, it had an aftermarket alarm. I don't recall a battery unit.
I choose to remove all components wich cost me remote locking, but I thought it would be good for battery life. Years later I had to repair a small electricity cable (part of a big bundle between the battery and the windscreen) due to on going wear, originally caused by wiretapping for the car alarm.

Most of the folks on here have UK spec cars, which don't have the factory fitted alarm but instead aftermarket alarms, either Scorpion 918U or Meta M99T2, were fitted at point of import to the UK.

Maybe sone fellow German Coopsters will be along to help...