How to replace control panel on old veritas 8 home alarm?

Hi all I have a really old version of the Veritas 8 alarm (no remote key pad facility) and so to be able to conneect up remote key pad I have purchased a new control panel, in the hope that I can simply replace the exisiting one by connecting up the current wiring on the new panel. the problem is when i remove the panel as stated in the install manual - using engineer code- I am surprised by that fact the external siren goes off even though I have removed mains power and back up battery. How is this possible where is the external box getting power, and is it possible to disable this to allow me to rewire the new control panel?