Replacement siren wiring

bought a car recently with a cobra alarm fitted, but no siren.  The alarm seems to work fine, arms disarms, flashes when a door is opened or the alarm is triggered by the sensors etc,  just no siren.

Anyway, there are 3 wires, one red, one black and one blue all blanked off next to a mount which is siren sized in the engine bay.

I have bought a back up battery siren with 4 wires on, red positive, black ground, blue neg trigger, white pos trigger.

The wires in the car are red = live, checked with multimeter, black = earth, and the blue one has a constant signal running through it (not 12v) whether the alarm is armed or dissarmed.

When I connect the live to live, ground to ground, and the pos trigger to this blue wire the alarm sounds constantly whether armed or dissarmed. If i connect the neg trigger to it it seems to work at first, chirps when i arm or dissarm, but after about 30 seconds starts to chirp repeatedly, but the chirp isn't a solid sound, a bit muffled.

Please help, its probably something simple.