Car alarm fitting help

Hi everyone,
I don't suppose there is a car alarm fitter among us..?

I'm trying to fit a Toad Ai606 to my land Rover 110, so far Ive got the main unit working but I cant get any noise out of the siren.

So far I've wired the unit to a good perm live, ignition live, a good earth and wired the indicator feed.
I've positioned the ultrasonic sensors and plugged them in.
I've positioned the siren under the bonnet and fed the wires through the bulkhead and connected them to the multi plug.
I've positioned the bonnet pin and plugged it in.
Bearing in mind I've fitted the alarm to is a 1988 Land Rover, and it doesn't have central locking or electric windows, it doesn't even have door switches...
I haven't connected the immobiliser yet...
The problem I'm getting is I have no siren when the alarm is triggered, for instance the system is armed and you open the door, my indicators flash to suggest the alarm has been set off but I have no sound.
Also last night for no apparent reason the siren went off, NOT the main alarm so no flashing indicators just the siren, I re-checked the connections at the multi plug, and all seems good at the moment...
As for the immobiliser, should that act was a switch..? So when the alarm is armed the alarm unit opens the circuit...? As when I checked the operation with my multi meter I had continuity all the time and no open circuit to suggest the immobiliser was not arming...
I get the quick green LED flash and slow flashing red LED.
When entering the pin code I hear no chirps from the siren to tell me if I've entered the code correctly...
Could the unit be faulty...?

Please help...