I agree with Eleckie Doc, probably the best solution is to have a new system installed.
I've had a similar problem recently, My remote plip buttons perished, I bought a new case for it and after a short while it just stopped working. Although I can't see anything wrong with the Plip it cannot be re synced to the car and Citroen want silly money for a new one and to re sync to the car so I just bought a new aftermarket unit with 2 Plips.
If you already have an alarm system most of the wiring should already be in place for upgrading to a new system.
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