Alarm/immobiliser

[luciann22]

Hi all

We have converted our 03 Belingo into a miro camper, however an awful screechimg alarm sound goes off every morning when we open the doors.
The screech comes through the speakers and goes off even if we have pressed the lock button inside the Berlingo.
When you put the engine on the noise reduces but its still coming through the speakers and stops the radio working. We have to either leave the engine running for around 10 minutes or turn it on and off around 3 tines.
Does anyone know what is causing this and how to stop it?
The garage told us to buy a new key, which we did but that hasn't helped!

Thanks