Help with external alarm siren modification

Hi.
I would like to make a simple DIY alarm system for a basement. I already have an external siren from taken from a burglar alarm system and i want to modify it for this project. My idea is to replace its own wall tamper microswitch with a wired magnetic sensor so every time the door opens to trigger the siren's alarm circuit. I replaced the internal 12v battery with a 12v dc transformer but when I open the microswitch nothing happened. The internal circuit has the following pinouts : the tamper microswitch, the + and + battery plugs, an on-board the circuit the tamper in and out (i think for the cable that runs to the main alarm board that Im not going to use one so I sort circuit it) +14v ,GND, GO-,GO+ and FT- .
Can anyone help me?
Thanks in advance and excuse my bad english.

Where did you get a DC transformer? Buy all you can and resell them for megabucks. You'll have the market cornered on DC transformers!

You dont need high IQ to understand that I meant ac-to-dc transformer my friend.
I suppose that with your comment I will finish my project. Thanks.

Many alarm "sirens" are not actually sirens but 8^ horn speakers & the alarm has an onboard driver.

Hi,

how about posting a circuit diagram for real help?

Boncuk