Disabling Focus car alarm

I've got a 1999 1.8i LX 5-door Focus. It came with the Ford built in car alarm. Has been fine for years, but recently, has been going off fairly
regularly without reason - sometimes at 3am - which makes me unpopular with the Wife and the Neighbours.
Does anyone know where the brain/box-of-tricks is for the Alarm is located ? (and what it looks like !) My Haynes manual doesn't offer any clues - I want to temporarily disable the Ultrasonics since I think it is them which is causing me problems.
At the moment - I have to disconnect the battery before going to bed to prevent any problems during the night - this is getting to be a lot of a pain !
Any help much appreciated. Cheers, Tim.
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