515U Smart Siren Charging

Hi all Long time no post, used to install these alarms and lots of other goodies for many years, however lost al my paperwork and notes accumulated over the years of installing so this may be a daft question now. the Siren on my old Show/Demo car has started going of randomly on its own no alarm trigger and no lights flashing and wont stop with lock unlock etc so this points at the siren being at fault. as the siren has been installed in the car since 2013 figured it wont hurt to replace it now. however as the car lives most its life of the road on a trickle charge this hasn't helped the original siren battery and don't want the same occurring for the new one now i know the car has to be driven for the backup battery to charge up so my question is how can i charge this siren without driving the car what tells the siren to use power from the running car vs the battery. there are 4 wires red & black raw 12v Power yellow and green to the service port does the alarm tell the siren its running and thus to switch on the charging circuit or does it auto sense the change in voltage to the 14v and above etc if it picks up the change in state can this be "spoofed" to enable an override to make it charge